Reject incompatible FTS and literal search modes

[Widthdom]

Summary

• Reject raw FTS combined with --exact, --exact-substring, or --token-boundary after argument normalization and before search dispatch.
• Emit typed E010_USAGE_ERROR responses for human and JSON search output while preserving the audit invocation contract.
• Keep replay/query context single-mode, and update help, completion snapshots, bilingual documentation, tests, and the changelog fragment.

Root cause

Exact-search aliases were normalized without a compatibility check against raw FTS. The token-boundary conflict used a separate late validation path, so incompatible semantic flags could reach query context together.

Validation

• dotnet build CodeIndex.sln -c Release — passed with 0 warnings/errors.
• Focused Issue Reject incompatible FTS and literal search modes #4879 and integration coverage — 10/10 passed on net8.0 and net9.0.
• Reviewer-selected search/help suites — 277/277 passed on net8.0 and net9.0.
• Full net8.0 suite — 10,814 passed, 7 skipped, 1 known MCP concurrency/load failure tracked by Stabilize MCP batch timeout concurrency lease test under full-suite load #4935 and Stabilize the batch timeout concurrency test under parallel suite execution #4947.
• Full net9.0 suite — 10,358 passed, 420 skipped, 3 failures: the known MCP concurrency/load failure, Stabilize cancelled EOF drain timing on .NET 9 #4978, and an audit error-context integration regression that was subsequently fixed and revalidated on net8.0/net9.0.
• dotnet run --project tools/CodeIndex.Changelog -- check — validated 53 fragments.
• Codex adversarial review — no blocking or actionable findings.

Related issues

• Stabilize MCP batch timeout concurrency lease test under full-suite load #4935
• Stabilize the batch timeout concurrency test under parallel suite execution #4947
• Stabilize cancelled EOF drain timing on .NET 9 #4978

Fixes #4879