Add bounded MCP language catalog pagination

[Widthdom]

Summary

• Reuse the canonical CLI/MCP language capability catalog for exact language, extension, alias, and capability filters.
• Add stable catalog-generation-bound cursor pagination with separate catalog, matched-catalog, indexed-workspace, returned, and remaining counts.
• Enforce maxBytes against the complete UTF-8 JSON-RPC response envelope and report whether byte budget or item limit caused continuation.
• Reject malformed, query-mismatched, and stale cursors with typed diagnostics while preserving schema-valid duplicate capability filters idempotently.

Validation

• dotnet restore CodeIndex.sln
• dotnet build CodeIndex.sln -c Release --no-restore -p:UseSharedCompilation=false — succeeded with 0 warnings and 0 errors on the final merged HEAD.
• Final merged HEAD language-focused Release suite (Issue4895, Issue4896, McpToolContractTests, RunLanguages, and ToolsCall_Languages) — 70 passed on net8.0 and 70 passed on net9.0.
• Full Release suite before merging the latest origin/main — net8.0: 10,962 passed / 7 skipped / 0 failed on the clean rerun; net9.0: 10,487 passed / 420 skipped / 0 failed. The final merge overlap is covered by the final 70-test suite above.
• dotnet format CodeIndex.sln --verify-no-changes --no-restore --verbosity minimal
• dotnet run --project tools/CodeIndex.Changelog -- check — 72 fragments validated.
• git diff origin/main..HEAD --check
• Local CodeIndex refresh and status --check --json — index matches the final workspace with 0 warnings and 0 errors.

Adversarial review

• Completed the workflow maximum of two Codex adversarial review rounds.
• Round 1 identified duplicate capability filters being rejected; fixed by idempotent normalization and regression coverage.
• Round 2 identified incomplete CLI capability vocabulary in MCP; fixed by sharing all ten CLI capability values across validation and schema with regression coverage.

Documentation and changelog

• Updated USER_GUIDE.md, DEVELOPER_GUIDE.md, and TESTING_GUIDE.md in their English and Japanese sections.
• Added changelog.d/unreleased/4896.added.md.

Fixes #4896

Follow-ups

• None.