Honor end-of-options for option-like search queries

[Widthdom]

Summary

• Preserve the one-token -- literal-query boundary when bounded JSON response controls are classified.
• Keep escaped option-like queries out of response control, database-path, and cursor-fingerprint processing while continuing to parse later search options normally.
• Add regression coverage for bounded controls, DB/data-dir lookalikes, and cursor mismatch behavior.

Validation

• Exact reproduction: dotnet ./src/CodeIndex/bin/Debug/net8.0/cdidx.dll search --json -- --fields (exit 0)
• Issue-focused tests: .NET 8/9, 2 passed per target
• JsonEnvelopeWrapperTests: .NET 8/9, 27 passed per target
• Full test suite: .NET 8 — 12,061 passed, 9 skipped, 0 failed; .NET 9 — 11,576 passed, 431 skipped, 0 failed
• dotnet format CodeIndex.sln --verify-no-changes --no-restore
• dotnet run --project tools/CodeIndex.Changelog -- check
• Release and Debug solution builds: 0 warnings, 0 errors
• Root and workspace index health checks: healthy and matched to HEAD
• Two bounded Codex adversarial review rounds; all reported findings addressed

Documentation and changelog

• Updated the English and Japanese sections of USER_GUIDE.md and TESTING_GUIDE.md.
• Added changelog.d/unreleased/5208.fixed.md.

Follow-up candidates

None.

Fixes #5208