Fix fixture compile-log sharing races

[coneilen]

Summary

Make the Windows capture-test fixtures read compiler logs while inherited telemetry writers may still hold them open. Preserve the actual compile exit code and hash provenance from the same bytes read.

Changes

• Added a fixture-local share-tolerant log snapshot reader and used it for stdout/stderr in both harmless compiler fixtures.
• Reused the same stdout/stderr snapshots for the harmless-target provenance hashes; no production/runtime code or semantic test cases changed.

Test plan

The RED and GREEN checks were the isolated held-writer harnesses. The regression command ran the full direct Windows fixture suite; macOS make test and make check were not run.

RED: if(Test-Path -LiteralPath 'C:\Users\coneilen.copilot\session-state\b26d36e4-9e49-4aab-b321-260853848824\files\Issue833-launch-temp-r2'){throw 'Fresh launch TEMP already exists; stop'}; $null=[IO.Directory]::CreateDirectory('C:\Users\coneilen.copilot\session-state\b26d36e4-9e49-4aab-b321-260853848824\files\Issue833-launch-temp-r2'); $env:TEMP='C:\Users\coneilen.copilot\session-state\b26d36e4-9e49-4aab-b321-260853848824\files\Issue833-launch-temp-r2'; $env:TMP=$env:TEMP; pwsh -NoProfile -File 'C:\Users\coneilen.copilot\session-state\b26d36e4-9e49-4aab-b321-260853848824\files\Issue833-HeldWriterRepro.ps1' -Mode Red -Root 'C:\gc833-f9b20c-r2' -TempRoot 'C:\Users\coneilen\AppData\Local\Temp\gc833-f9b20c-r2' *>&1 | Out-File -FilePath 'C:\Users\coneilen.copilot\session-state\b26d36e4-9e49-4aab-b321-260853848824\files\Issue833-Red2-console.txt' -NoClobber -Encoding utf8; $reproExit=$LASTEXITCODE; Write-Output ("ISSUE833_RED_PROCESS_EXIT={0}" -f $reproExit); if($reproExit -ne 0){exit $reproExit} -> result.json status=RED, executed=1, passed=0, failed=1; ReadAllText HRESULT 0x80070020 while writer held; harness exit marker 0 after recording intended behavioral RED
GREEN: if(Test-Path -LiteralPath 'C:\Users\coneilen.copilot\session-state\b26d36e4-9e49-4aab-b321-260853848824\files\Issue833-launch-temp-g1'){throw 'Fresh launch TEMP already exists; stop'}; $null=[IO.Directory]::CreateDirectory('C:\Users\coneilen.copilot\session-state\b26d36e4-9e49-4aab-b321-260853848824\files\Issue833-launch-temp-g1'); $env:TEMP='C:\Users\coneilen.copilot\session-state\b26d36e4-9e49-4aab-b321-260853848824\files\Issue833-launch-temp-g1'; $env:TMP=$env:TEMP; pwsh -NoProfile -File 'C:\Users\coneilen.copilot\session-state\b26d36e4-9e49-4aab-b321-260853848824\files\Issue833-BoundGreen.ps1' -SourcePath 'D:\depot\copilot-worktrees\GraphCode\coneilen-issue-833-fix-fixture-compile-log-sharing-race-f9b20c\Tools\windows\Tests\UiCaptureDriver.Tests.ps1' -ExpectedSourceSha256 'DF842491992C6571142074B2686FF40E49FA51CC360F6108D31E6209C268F0A7' -Root 'C:\gc833-f9b20c-green-1' -TempRoot 'C:\Users\coneilen\AppData\Local\Temp\gc833-f9b20c-green-1' *>&1 | Out-File -FilePath 'C:\Users\coneilen.copilot\session-state\b26d36e4-9e49-4aab-b321-260853848824\files\Issue833-Green1-console.txt' -NoClobber -Encoding utf8; $reproExit=$LASTEXITCODE; Write-Output ("ISSUE833_GREEN_PROCESS_EXIT={0}" -f $reproExit); if($reproExit -ne 0){exit $reproExit} -> source-bound helper GREEN: executed=1, passed=1, failed=0; legacy ReadAllText HRESULT 0x80070020; tracked helper text and same-byte SHA-256 matched
REGRESSION: if(Test-Path -LiteralPath 'C:\Users\coneilen\AppData\Local\Temp\gc833-f9b20c-59'){throw 'Private TEMP already exists; stop'}; if(Test-Path -LiteralPath 'C:\gc833-f9b20c-59'){throw 'Full59 root already exists; stop'}; if(Test-Path -LiteralPath 'C:\Users\coneilen.copilot\session-state\b26d36e4-9e49-4aab-b321-260853848824\files\Issue833-59-console.txt'){throw 'Full59 console already exists; stop'}; $null=[IO.Directory]::CreateDirectory('C:\Users\coneilen\AppData\Local\Temp\gc833-f9b20c-59'); $env:TEMP='C:\Users\coneilen\AppData\Local\Temp\gc833-f9b20c-59'; $env:TMP=$env:TEMP; pwsh -NoProfile -File 'D:\depot\copilot-worktrees\GraphCode\coneilen-issue-833-fix-fixture-compile-log-sharing-race-f9b20c\Tools\windows\Tests\UiCaptureDriver.Tests.ps1' -ScratchRoot 'C:\gc833-f9b20c-59' *>&1 | Out-File -FilePath 'C:\Users\coneilen.copilot\session-state\b26d36e4-9e49-4aab-b321-260853848824\files\Issue833-59-console.txt' -NoClobber -Encoding utf8; $testExit=$LASTEXITCODE; Write-Output ("ISSUE833_FULL59_PROCESS_EXIT={0}" -f $testExit); exit $testExit -> exit marker 0; UiCaptureDriver executed=59, passed=59, failed=0; report shows 59 unique case names

Checklist

• I have read the Contributing Guidelines
• I have signed off my commits (git commit -s) per the DCO
• Tests pass locally (make test)
• Code follows the existing style (make check)
• I added the test/contract before the implementation and observed the intended RED failure

[coneilen]

Refs #833. The issue stays open until merge and coordinator verification. This comment is for linkage only.