Infra: Add CI/CD GitHub Actions workflows (modeled after puppeteer-sharp)

[kblok]

Description

Add GitHub Actions CI/CD workflows to PlaywrightSharp, modeled after puppeteer-sharp's pipeline.

What to Implement

Main test workflow (.github/workflows/dotnet.yml)

• Triggers: workflow_dispatch, push to main/release-*, PRs
• File filters: Only trigger on .yml, .cs, .json, .csproj, .runsettings changes
• .NET SDK: 10.0.x
• Matrix strategy:
  • OS: Ubuntu and Windows
  • Browsers: Chromium, Firefox (WebKit when ready)
  • Modes: headless, headful
• Steps:
  1. Checkout
  2. Setup .NET 10 SDK
  3. Install system dependencies (libgbm-dev, xvfb on Linux)
  4. Generate HTTPS dev certificate (dotnet dev-certs https -ep src/PlaywrightSharp.TestServer/testCert.cer)
  5. Code formatting check (dotnet format) — only on first matrix combo
  6. Puppeteer reference check (no "puppeteer" in code)
  7. Build solution
  8. Run tests with PRODUCT env var and --filter Category= flag
  9. Use xvfb-run on Linux for headful tests
• Test retry: Consider adding dotnet-retest for flaky browser test handling
• Timeout: TestSessionTimeout of 3600000ms via .runsettings

Code style workflow (.github/workflows/code-style.yml)

• Runs dotnet format --check
• Checks for "puppeteer" references in the codebase

NuGet publish workflow (.github/workflows/publish-nuget.yml)

• Triggers on version tags
• Publishes packages to nuget.org

Upstream References

• puppeteer-sharp .github/workflows/dotnet.yml
• puppeteer-sharp .github/workflows/code-style.yml
• puppeteer-sharp .github/workflows/PushNugetPackageToNugetOrg.yml

Files to Create/Modify

• .github/workflows/dotnet.yml (new)
• .github/workflows/code-style.yml (new)
• .github/workflows/publish-nuget.yml (new)
• src/PlaywrightSharp.Tests/test.runsettings (new)

Verification

Push a branch and verify workflows trigger and execute correctly.

Dependencies

Depends on: #1