Fix exact indexed file identity for LSP document symbols

[Widthdom]

Summary

LSP document-symbol requests could include declarations from a case-colliding or pattern-matching indexed path, corrupting the requested document's hierarchy and ranges. Indexed lookup and the unsupported-live-extraction fallback now apply binary-equal, parameterized indexed-path selection before ordering and candidate limits.

Regression coverage checks case collisions without case-sensitive storage, literal path characters, hierarchy and identifier ranges, framed partial-result parity, and foreign declarations exceeding the materialization limit without displacing local symbols or causing false truncation. Existing live-extraction, cancellation and response-budget coverage remains in the LSP regression run.

Validation

• New identity regression reproduced the foreign-root failure before the fix.
• dotnet build — passed.
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj --no-restore --filter 'FullyQualifiedName~Lsp|FullyQualifiedName~SearchSymbols' — net8: 253 passed; net9: 251 passed, 2 existing conditional skips.
• Real CLI indexing followed by initialized Content-Length-framed stdio LSP — indexed and unsupported-live-extraction fallback modes passed the issue reproduction.
• dotnet format CodeIndex.sln --verify-no-changes --no-restore — passed.
• dotnet run --project tools/CodeIndex.Changelog -- check — passed.
• Post-commit root status --check and workspace health checks — passed.
• Codex adversarial review of origin/main..HEAD — No blocking/actionable issues found.

Documentation

Updated both English and Japanese sections of DEVELOPER_GUIDE.md and TESTING_GUIDE.md. Added changelog.d/unreleased/5382.fixed.md; CHANGELOG.md is unchanged.

Fixes #5382