Add indexed LSP call hierarchy (#5351)

[Widthdom]

Summary

Adds standard indexed LSP call hierarchy preparation and incoming/outgoing expansion for #5351. Items bind exact symbol IDs to the session and index generation; repeated call sites retain UTF-16 ranges and valid file URIs. Incomplete, ambiguous, stale, unsaved or over-budget evidence returns explicit LSP errors, and successful results explain indexed-graph limitations.

The implementation uses existing persisted call identities across 13 languages without adding language binding or runtime dependencies.

Validation

• Release solution build succeeded with zero warnings/errors. Before the final review fixes and upstream merge, the full Release suite passed: net8.0 12,567 passed / 9 skipped; net9.0 12,073 passed / 435 skipped.
• After review fixes: 30 focused hierarchy tests passed on each framework. After merging the fetched main, the broader LSP regression filter passed again: 205 on net8.0 and 203 on net9.0 (2 existing skips).
• Framed real-index LSP fixtures covering identity, overloads, recursion, repeated/Unicode sites, language coverage, stale/unsaved/incomplete data, cancellation, and fan-in/fan-out limits.
• make lint, git diff --check, and dotnet run --project tools/CodeIndex.Changelog -- check.
• Codex adversarial review of origin/main..HEAD: fixed all three first-round findings and added regressions for response-budget session reuse, exact file identity and bounded overlapping-definition metadata. The second review concluded: No blocking/actionable issues found.
• Public stdio LSP smoke against the repository index verified preparation, exact incoming callers and explicit unresolved-outgoing rejection.
• Rebuilt the local CLI and refreshed the canonical root index; root and workspace-manifest checks are healthy, with workspace, index and reference-graph completeness verified at the final commit.

Documentation

Updated the English and Japanese sections of USER_GUIDE.md, DEVELOPER_GUIDE.md and TESTING_GUIDE.md, with the detailed contract in docs/lsp-call-hierarchy.md.

Changelog fragment: changelog.d/unreleased/5351.added.md.

Follow-ups: #5362, #5363, #5373.

Fixes #5351