Fix C# symbol extraction edge cases

[Widthdom]

Summary

• classify C# const and static readonly declarations as field symbols
• preserve real method names for multiline tuple return types
• lock down existing suppression of multiline out modifiers, static lambdas, and raw-string SQL contents with focused regression tests

Root cause and impact

The field patterns historically used the function extraction path, and multiline tuple return headers were not recognized when the opening tuple parenthesis ended the modifier line. This polluted symbol kinds or lost the real method name. The updated classification preserves the mature scanning path while correcting the emitted kind, and the tuple-prefix recognizer lets the existing bounded header merger assemble the declaration correctly.

Validation

• dotnet run --project tools/CodeIndex.Changelog -- check
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false
• dotnet test CodeIndex.sln -c Release -p:UseSharedCompilation=false --no-build
• SymbolExtractor suite on net8/net9
• focused tests for issues 4405, 4413, 4453, 4454, and 4456 on net8/net9
• Codex adversarial review: No blocking/actionable issues found.

Documentation and changelog

Bilingual changelog fragments:

• changelog.d/unreleased/4405.fixed.md
• changelog.d/unreleased/4413.fixed.md
• changelog.d/unreleased/4453.fixed.md
• changelog.d/unreleased/4454.fixed.md
• changelog.d/unreleased/4456.fixed.md

No standalone documentation contract changed.

Follow-up candidates

None.

Fixes #4405
Fixes #4413
Fixes #4453
Fixes #4454
Fixes #4456