Add --list-tests json for machine-readable test discovery output

[Evangelink]

Adds an optional json argument to the existing --list-tests flag so MTP can emit a single JSON document on stdout describing every discovered test. The text output and zero-arg behavior are unchanged.

Fixes #3221.
Related: dotnet/sdk#49754.

CLI surface

Command	Behavior
--list-tests	Unchanged (human-readable text)
--list-tests text	Explicit alias for the default
--list-tests json	JSON document on stdout (banner / progress / summary / per-test text suppressed; errors routed to stderr)
--list-tests <anything-else>	Fails with ExitCode.InvalidCommandLine and a descriptive validation error

Schema v1

Flat array; absent fields are omitted (no null), so frameworks that don't populate TestMethodIdentifierProperty aren't broken.

{
  ""schemaVersion"": 1,
  ""tests"": [
    {
      ""uid"": ""..."",
      ""displayName"": ""..."",
      ""type"": {
        ""assemblyFullName"": ""..."",
        ""namespace"": ""..."",                  // omitted for global namespace
        ""typeName"": ""MyClass+Nested`1"",      // metadata format with arity & nesting
        ""methodName"": ""MyMethod"",
        ""methodArity"": 0,
        ""returnTypeFullName"": ""..."",
        ""parameterTypeFullNames"": [""..."", ...]
      },
      ""location"": { ""file"": ""..."", ""lineStart"": 12, ""lineEnd"": 18 },
      ""traits"":     [ { ""key"": ""Category"", ""value"": ""Integration"" }, ... ],
      ""properties"": [ { ""key"": ""ExecutionId"", ""value"": ""..."" }, ... ]
    }
  ]
}

traits and properties are both arrays of { key, value } (not JSON objects) so duplicate keys survive serialization and order is stable across runs.

Implementation notes

• New DiscoveredTestsJsonSerializer + tiny JsonStringWriter — no dependency on System.Text.Json (not available across all targets) or Jsonite (#if !NETCOREAPP only), so the same code path runs on netstandard2.0, net462, net8.0, net9.0.
• Always-LF line endings for stable CI golden output, U+2028/U+2029 escaped, lone surrogates replaced with U+FFFD (mirrors Utf8JsonWriter defaults).
• TerminalOutputDevice detects JSON mode, suppresses banner / progress / per-test text / free-form output on stdout, buffers DiscoveredTestNodeStateProperty events from ConsumeAsync, and emits the JSON document once at session end from the test-host process only (controller does not double-emit).
• Ctrl+C in JSON mode writes the cancellation message to stderr instead of corrupting the JSON document.
• PlatformCommandLineProvider flips --list-tests arity from Zero to ZeroOrOne, validates the argument against json/text (case-insensitive), and exposes IsListTestsJsonOutput(...).

Tests

Suite	Result
Microsoft.Testing.Platform.UnitTests (full, net462 / net8 / net9)	2,277 / 2,277 (159 in new/touched test classes)
MSTest.Acceptance.IntegrationTests.TestDiscoveryTests (incl. new JSON tests × 2 TFMs)	12 / 12
MSTest.Acceptance.IntegrationTests.HelpInfoTests	6 / 6
Microsoft.Testing.Platform.Acceptance.IntegrationTests.HelpInfoTests*	30 / 30
Microsoft.Testing.Platform.Acceptance.IntegrationTests.ExecutionTests	27 / 27
Microsoft.Testing.Platform.Acceptance.IntegrationTests.TrxTests	28 / 28
.\build.cmd -pack -c Debug (warnings-as-errors)	succeeded, 0 warnings, 0 errors

Notes

• No public API surface added (option key is internal CLI only).
• PlatformResources.resx updated; all 13 XLFs regenerated via dotnet msbuild … /t:UpdateXlf.
• No changelog entry — docs/Changelog-Platform.md is curated at release time, and v2.2.3 was just cut without an Unreleased section.

The change went through two rounds of internal expert review before opening. Highlights addressed in review:

• Errors no longer silently swallowed in JSON mode (routed to stderr).
• Ctrl+C no longer corrupts the JSON document (cancellation message goes to stderr).
• PropertyBag's reverse-insertion-order traversal is reversed back to insertion order.
• properties is an array (not object) so duplicate keys are preserved.
• type.namespace omitted for the global namespace.
• --list-tests text accepted as explicit alias of the default for forward compatibility.

[Copilot]

Pull request overview

Adds machine-readable JSON output for --list-tests while preserving the existing text behavior.

Changes:

• Adds json/text argument handling and validation for --list-tests.
• Adds JSON discovery serialization and suppresses normal terminal output in JSON mode.
• Adds unit and acceptance coverage plus updated help/resource localization files.

Show a summary per file

File	Description
src/Platform/Microsoft.Testing.Platform/CommandLine/PlatformCommandLineProvider.cs	Allows optional --list-tests output format and validates accepted values.
src/Platform/Microsoft.Testing.Platform/OutputDevice/TerminalOutputDevice.cs	Buffers discovered tests and emits JSON-only stdout in JSON mode.
src/Platform/Microsoft.Testing.Platform/OutputDevice/DiscoveredTestsJsonSerializer.cs	Serializes discovered test nodes into schema v1 JSON.
src/Platform/Microsoft.Testing.Platform/OutputDevice/JsonStringWriter.cs	Adds a small JSON writer for cross-target serialization.
src/Platform/Microsoft.Testing.Platform/Resources/PlatformResources.resx	Updates CLI help text and invalid argument resource.
src/Platform/Microsoft.Testing.Platform/Resources/PlatformResources.cs	Exposes the new validation resource to unit tests.
src/Platform/Microsoft.Testing.Platform/Resources/xlf/*	Regenerates localized resource entries.
test/UnitTests/Microsoft.Testing.Platform.UnitTests/CommandLine/PlatformCommandLineProviderTests.cs	Covers list-tests argument validation and JSON mode detection.
test/UnitTests/Microsoft.Testing.Platform.UnitTests/OutputDevice/DiscoveredTestsJsonSerializerTests.cs	Covers JSON schema, escaping, ordering, and optional fields.
test/IntegrationTests/MSTest.Acceptance.IntegrationTests/TestDiscoveryTests.cs	Adds acceptance coverage for JSON discovery output and invalid format.
test/IntegrationTests/*/HelpInfo*.cs	Updates help/info expectations for the new option description and arity.

Copilot's findings

• Files reviewed: 25/25 changed files
• Comments generated: 1

[Evangelink]

Review Summary — PR #8280: --list-tests json

Overall this is a well-structured, well-tested feature. The custom JsonStringWriter, cross-TFM compatibility, lone-surrogate handling, banner/progress suppression, and CLI validation are all cleanly implemented. Tests cover the happy path, special character escaping, and invalid-argument rejection.

Findings

#	Dimension	Severity	File	Finding
1	Threading/Hot-reload	MODERATE	TerminalOutputDevice.cs:379	DisplayAfterHotReloadSessionEndAsync calls DisplayAfterSessionEndRunInternalAsync unconditionally; in a dotnet watch --list-tests json session the JSON document is emitted after every cycle and _discoveredTestsForJson is never cleared — multiple growing JSON blobs on stdout.
2	Testability/Architecture	NIT	TerminalOutputDevice.cs:420	Console.Error.WriteLineAsync is used directly (also in the abort callback in InitializeAsync) bypassing the injected _console. This makes error/stderr output in JSON mode un-redirectable in tests. The comment acknowledges the limitation; worth tracking.

Clean dimensions

• Algorithmic Correctness — JSON writer logic, escaping, surrogate pair handling all correct.
• Threading — _discoveredTestsForJson write path is guarded by MTP's single-consumer-per-pump invariant; the emission path uses _asyncMonitor. Correct.
• Security — No path traversal, no serialization of untrusted schemas, no sensitive leakage.
• Public API — No new public API surface; IsListTestsJsonOutput is internal static. PublicAPI.Unshipped.txt not dirtied.
• Cross-TFM — Custom JsonStringWriter avoids both System.Text.Json and Jsonite; no #if-guarded APIs needed.
• Performance — StringBuilder-based writer; no hot-path LINQ; PropertyBag.OfType<T> returns an array, Array.Reverse is in-place. Reasonable.
• CLI validation — Argument validated in ValidateOptionArgumentsAsync; ZeroOrOne arity correct; case-insensitive comparison correct; resource string properly parametrised.
• Localization — New strings in .resx; XLFs regenerated via MSBuild, not hand-edited.
• Test quality — Unit tests cover empty, minimal, full, special-char, surrogate, and validation cases. Integration tests cover JSON output schema, invalid arg, and unchanged text mode.
• Assertion style — Uses MSTest conventions consistently throughout.
• Scope discipline — PR is tightly scoped to the new --list-tests json flag; no unrelated refactoring.

Generated by Expert Code Review (on open) for issue #8280 · ● 19.8M

[Evangelink]

Pushed 884c759 to address review comments and unblock CI:

• Hot-reload double-emission (Evangelink, MODERATE) — DisplayAfterHotReloadSessionEndAsync now early-returns in JSON mode so dotnet watch --list-tests json doesn't emit multiple ever-growing JSON documents.
• Console.Error bypass (Evangelink, NIT) — both call sites now route through a single WriteToStandardErrorAsync helper with a comment explaining it's the sole place that bypasses IConsole until IConsole gains stderr support.
• Help text (Copilot) — the description now mentions both text and json.
• CI XLF failure — ExtensionResources.pl.xlf (TRX) was reverted to its pre-Support placeholders in --report-trx-filename #8223 source during my earlier rebase (I discarded it thinking it was unrelated drift; it was actually a translation update that had to stay). Restored from origin/main.

Help/info acceptance tests updated for the new description; XLFs regenerated; .\build.cmd -pack -c Debug passes with 0 warnings, 0 errors; unit tests 159/159 in the touched classes.

[Copilot]

Copilot's findings

• Files reviewed: 26/26 changed files
• Comments generated: 0 new