fix: include XML documentation files in NuGet packages

[thomhurst]

Summary

• Enable GenerateDocumentationFile in Library.props so all library projects generate XML doc files
• XML doc files are automatically included in NuGet packages by SDK convention, providing IntelliSense for consumers
• Suppress XML doc warnings (CS1591, CS1570, etc.) since not all public APIs are documented yet and TreatWarningsAsErrors is enabled

Closes #5563

Test plan

• TUnit.Core builds clean with XML doc generation enabled
• TUnit.Assertions builds clean
• TUnit.Engine builds clean
• Verified TUnit.Core.xml file generated in output directory

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[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
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[claude]

Code Review

This is a clean, minimal fix for issue #5563. The approach — enabling GenerateDocumentationFile globally in Library.props while suppressing documentation-related warnings — is exactly right for a project of this scale that has partial XML doc coverage. Here are a few observations:

What's good

• Correct placement. Library.props is the right file; all 15+ packable library projects import it, so the XML files will be generated and automatically included in NuGet packages by SDK convention without any additional <None> packaging entries.
• $(NoWarn) accumulation is correct. Using $(NoWarn);... preserves existing suppressed codes (NU1507;NU1903;CS9107) rather than overwriting them.
• Reasonable warning list. All 10 suppressed codes (CS1591, CS1570, CS1572, CS1573, CS1574, CS1584, CS1587, CS1658, CS1734, CS0419) are XML-documentation-specific — nothing unrelated is being silenced.

Minor observations

1. CS1570 is fairly broad. It covers malformed XML in doc comments, which includes genuinely broken tags. It is suppressed globally here, so any new malformed comment won't produce a diagnostic. This is acceptable given TreatWarningsAsErrors is on and full doc coverage isn't the goal yet, but it's worth tracking as a future cleanup target.

2. Non-packable projects also get GenerateDocumentationFile=true. Projects like TUnit.Engine (which has IsPackable not explicitly set, so it defaults to the SDK's IsTestingPlatformApplication-driven logic) and potentially others will also generate XML files on build. These files are harmless and won't be packed if the project isn't packable, but it does add minor I/O to every build. A Condition="'$(IsPackable)' != 'false'" guard on GenerateDocumentationFile would prevent generating docs for internal-only projects, though this is a very minor concern.

3. Analyzer/source-generator projects are unaffected. They use Roslyn.props rather than Library.props, so their build is unchanged — good.

4. No snapshot test impact expected. This change is purely in MSBuild configuration and doesn't touch source generation or public API surface, so no .verified.txt files need updating.

Bottom line

The change is correct and complete for its stated purpose. The suppressed warning list is appropriately scoped. The only meaningful follow-up consideration would be eventually removing suppressions incrementally as XML docs are added (CS1591 being the highest-value one to eventually enforce), but that's a separate, larger effort — not a blocker for this PR.