Reject C# continuation lines as declarations

[Widthdom]

Summary

• add a column-aware declaration-start scope for C# function candidates
• reject invocation and parameter continuation lines while preserving valid multiline methods, constructors, delegates, and local functions
• reconcile preprocessor branch state and ignore directive payload delimiters
• bump the C# symbol extractor contract to 7

Root cause

The broad multiline declaration merger could begin inside an unmatched expression or parameter parenthesis and consume a later brace, turning continuation tokens into function declarations.

Validation

• dotnet build CodeIndex.sln -c Release --no-restore -p:UseSharedCompilation=false
• dotnet build CodeIndex.sln -c Debug --no-restore -p:UseSharedCompilation=false
• full Release suite after the final semantic fix:
  • net9.0: 10,039 passed, 419 skipped, 0 failed
  • net8.0: 10,512 passed, 7 skipped, 0 failed
• focused C# and Issue Reject C# continuation lines as function declarations #4831 tests after merging the latest origin/main:
  • net9.0: 292 passed, 5 skipped, 0 failed
  • net8.0: 297 passed, 0 skipped, 0 failed
• rebuilt the repository index and verified exact function queries:
  • GetIndexedLanguageCount: one real declaration
  • out: zero declarations
  • params: zero declarations
• dotnet format CodeIndex.sln --no-restore --verify-no-changes
• dotnet run --project tools/CodeIndex.Changelog -- check
• cdidx status --check --json: index matches the workspace

Documentation and changelog

• updated TESTING_GUIDE.md in English and Japanese
• added changelog.d/unreleased/4831.fixed.md

Review

Two Codex adversarial review rounds were completed. All reported actionable findings were addressed; the configured two-round review limit was reached.

Follow-ups

None.

Fixes #4831