feat: add GitHub Actions CI/CD workflows and local test runner

[gragra33]

Summary

Adds full GitHub Actions CI/CD automation for the Utf8JsonAsyncStreamReader project.

Changes

.github/workflows/ci.yml

• Triggers on push to feature/**, fix/**, hotfix/** and PRs to develop / master
• validate-branch job: enforces feature/, fix/, or hotfix/ prefix on PR source branches
• build-and-test job: matrix across net8.0, net9.0, net10.0 with all three SDKs installed
• Uploads .trx test result artefacts (with continue-on-error: true for known act local limitation)

.github/workflows/release.yml

• Triggers on push to master
• Builds and tests all three target frameworks
• Extracts version from System.Text.Json.Stream.csproj
• Guards against re-releasing an already-tagged version
• Packs Utf8JsonAsyncStreamReader.nupkg + .snupkg (symbols enabled)
• Creates a GitHub Release via gh CLI with both artefacts attached
• Pushes .nupkg and .snupkg to NuGet.org via ${{ secrets.NUGET_API_KEY }}

src/System.Text.Json.Stream.CI.slnf

• Solution filter scoping CI to library + tests only
• Excludes System.Text.Json.Stream.Benchmarks and System.Text.Json.Stream.Benchmarks.Old

ci-cd-test-run.ps1

• Local runner: lint (actionlint), dry (act -n), ci (full act), all modes
• Supports -Workflow ci|release|both and -Job <name> targeting
• Handles known act Windows artifact-cache cleanup false-positive

Pre-merge validation

• ✅ Build: passed (Release configuration)
• ✅ Tests: 43/43 passed on net8.0, net9.0, net10.0
• ✅ Pack: Utf8JsonAsyncStreamReader.2.1.0.nupkg + .snupkg produced
• ✅ Dry-run CI workflow: passed
• ✅ Dry-run Release workflow: passed
• ✅ actionlint: zero violations

Required setup (one-time, before first release)

Add NUGET_API_KEY as a repository secret:
Repository → Settings → Secrets and variables → Actions → New repository secret

[Copilot]

Pull request overview

Adds CI and release automation for the Utf8JsonAsyncStreamReader repo via GitHub Actions, plus a local PowerShell runner to lint/dry-run/execute workflows using actionlint + act.

Changes:

• Introduces CI workflow with PR branch-name validation and a test matrix across net8.0/net9.0/net10.0.
• Introduces release workflow that builds/tests, packs, creates a GitHub Release, and pushes packages to NuGet.
• Adds a CI-focused solution filter and a local CI/CD runner script.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 5 comments.

File	Description
src/System.Text.Json.Stream.CI.slnf	Adds a solution filter to scope CI builds/tests to the library + tests.
ci-cd-test-run.ps1	Adds a local runner for actionlint + act modes (lint/dry/ci/all) with some output parsing.
.github/workflows/ci.yml	Adds CI workflow with branch validation and a TFM matrix test run plus artifact upload.
.github/workflows/release.yml	Adds release workflow for master pushes to publish NuGet packages and create GitHub releases.

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