Update P/Invoke skill for cross platform declaration

[huoyaoyuan]

Add cases for default naming convention.
Prefer OperatingSystem api over OSPlatform api.

@AaronRobinsonMSFT

[github-actions]

Skill Validation Results

Skill	Scenario	Quality	Skills Loaded	Overfit	Verdict
dotnet-pinvoke	Generate LibraryImport declaration from C header (.NET 8+)	5.0/5 → 5.0/5	✅ dotnet-pinvoke; tools: skill	✅ 0.11	✅
dotnet-pinvoke	Generate LibraryImport declaration from C header (.NET Framework)	4.3/5 → 5.0/5 🟢	✅ dotnet-pinvoke; tools: skill	✅ 0.11	✅ [1]

[1] ⚠️ High run-to-run variance (CV=90%) — consider re-running with --runs 5

Model: claude-opus-4.6 | Judge: claude-opus-4.6

🔍 Full Results - additional metrics and failure investigation steps

▶ Sessions Visualisation -- interactive replay of all evaluation sessions
📊 Session Analytics (preview) -- aggregated metrics across evaluation sessions

[github-actions]

✅ Evaluation passed for 13fd251. cc @dotnet/appmodel @dotnet/skills-csharp-language-reviewers — please review.

[github-actions]

✅ Approved by @AaronRobinsonMSFT. cc @dotnet/skills-merge-approvers — ready to merge.