Fix C# constructor navigation identity filtering

[Widthdom]

Summary

• Resolve C# instantiate references by exact source site, generic type arity, and invocation arity.
• Keep explicit and primary constructor candidates separate from same-named partial type declarations.
• Preserve implicit/default construction for partial classes, value types, enums, and delegates while excluding static constructors and finalizers.
• Return every matching same-arity overload from LSP definition/declaration requests without falling back to the type family.
• Bump the retained reference identity contract so existing indexes rebuild affected candidates automatically.

Root cause

Constructor references and type declarations shared a name-based candidate path. In partial types this allowed a same-file type declaration or an unrelated constructor shape to outrank the callable target, while LSP position resolution could then collapse the mixed candidate set to the wrong location.

Validation

• dotnet build CodeIndex.sln -c Release --no-restore — passed with 0 warnings and 0 errors.
• Release Issue Resolve constructor navigation by symbol identity instead of partial-type name #4850 tests on net8.0 and net9.0 — 15/15 passed on each framework.
• Full Release suite on net9.0 — 10,175 passed, 419 skipped, 0 failed.
• Full Release suite on net8.0 — 10,647 passed, 7 skipped, with one unrelated MCP timeout fixture failure under parallel load; the exact failed test passed 1/1 when rerun alone.
• dotnet format CodeIndex.sln --no-restore --verify-no-changes — passed.
• dotnet run --project tools/CodeIndex.Changelog -- check — validated 26 fragments.
• Local index delta refresh and status --check --json — index_matches_workspace: true, no failed checks.

Documentation and changelog

• Updated the English and Japanese constructor-navigation sections in USER_GUIDE.md.
• Updated the English and Japanese regression-maintenance guidance in TESTING_GUIDE.md.
• Added changelog.d/unreleased/4850.fixed.md.

Follow-ups

None.

Fixes #4850