Fix #361: add regression coverage for split-line C# return types

[Widthdom]

Summary

• add explicit SymbolExtractor regression coverage for C# members whose return type is on the previous line
• assert the full non-import symbol set from the issue C# methods/properties/indexers with return type on a separate line from the name are silently dropped — per-line regex cannot match split signatures #361 repro, including the Create<T> control case
• verify the expected line ranges, visibility, and return types for the split-line method/property/indexer shapes

Verification

• dotnet test
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter Extract_CSharp_SplitReturnTypeLine_StillCapturesMethodPropertyAndIndexer
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --filter FullyQualifiedName~CodeIndex.Tests.SymbolExtractorTests
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll index <tmpdir> --json
• dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll symbols --db <tmpdir>/.cdidx/codeindex.db --json --lang csharp

Notes

• adversarial codex review reported no blocking/actionable findings after the follow-up test tightening
• filed follow-up issue C#: local variable initializers and multiline call arguments can be misclassified as phantom function definitions (content, FromSeconds) #496 for an unrelated C# phantom-function false positive found during review

Fixes #361