Add multi-Roslyn configuration support in DocoptNet.csproj

[Copilot]

• Modify src/DocoptNet/DocoptNet.csproj:
  • Add _Roslyn44AnalyzerDir property for path reuse
  • Add PropertyGroup conditioned on RoslynVersion=4.4 with isolated BaseOutputPath, BaseIntermediateOutputPath, DefaultItemExcludes, and ROSLYN4_4 define constant
  • Split Microsoft.CodeAnalysis.CSharp reference into two conditioned references (3.10.0 baseline vs 4.4.0)
  • Add second analyzer pack item for analyzers/dotnet/roslyn4.4/cs
  • Use versioned analyzers/dotnet/roslyn3.10/cs for baseline (instead of unversioned analyzers/dotnet/cs) to prevent duplicate loading
  • Add _ValidateRoslyn44AnalyzerOutput guard target before GenerateNuspec
• Update .github/workflows/ci.yml with Roslyn 4.4 variant build step
• Fix duplicate source generator loading on Linux CI (analyzers placed in versioned roslyn3.10/ and roslyn4.4/ folders)
• Verify baseline dotnet build still works (0 warnings, 0 errors)
• Verify dotnet build -f netstandard2.0 -p:RoslynVersion=4.4 works with isolated output
• Verify dotnet pack fails with guard message when Roslyn 4.4 output is absent
• Verify dotnet pack succeeds with both variants and nupkg contains correct structure
• Verify existing tests pass (756 tests passed)
• Verify integration tests pass on both net8.0 and net10.0 (72 tests each)

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[atifaziz]

Check the latest workflow run for this PR that's failing and fix the problem. The build is failing on the Linux image.

[Copilot]

Check the latest workflow run for this PR that's failing and fix the problem. The build is failing on the Linux image.

The Linux CI was failing because the integration test (tests/Integration/run.ps1) runs dotnet pack without -NoPack, which triggers the _ValidateRoslyn44AnalyzerOutput guard target — but the Roslyn 4.4 variant was never built in CI.

Fixed in 924ba4d by adding a "Build Roslyn 4.4 variant" step to the CI workflow that runs dotnet build src/DocoptNet/DocoptNet.csproj -f netstandard2.0 -p:RoslynVersion=4.4 after the baseline build and before pack/integration tests.

[atifaziz]

Check the latest workflow run for this PR that's still failing and it's still the Linux image. Test your changes before claiming your fix is good and keep working until resolved or you've had a roadblock.

[Copilot]

Check the latest workflow run for this PR that's still failing and it's still the Linux image. Test your changes before claiming your fix is good and keep working until resolved or you've had a roadbl...

The root cause was that the NuGet SDK loads both unversioned (analyzers/dotnet/cs/) and versioned (analyzers/dotnet/roslyn4.4/cs/) analyzer DLLs simultaneously — per the spec, unversioned analyzers are "considered to work everywhere." This caused the source generator to run twice, producing duplicate type definitions.

Fixed in cbefade by placing the baseline analyzer in analyzers/dotnet/roslyn3.10/cs/ instead of the unversioned analyzers/dotnet/cs/. Now the SDK selects only the best-matching versioned folder (roslyn4.4 for SDK >= 4.4, roslyn3.10 for older SDKs). Verified locally: baseline build, unit tests (756 passed), and full integration tests with pack (72 tests passed on both net8.0 and net10.0).

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