Require safe paths for summarize_file MCP prompt

[Widthdom]

Summary

• Mark the summarize_file prompt's path argument as required in prompt discovery.
• Return structured JSON-RPC -32602 errors for missing, null, non-string, blank, absolute, drive-prefixed, control-character-containing, or traversing paths instead of rendering an unresolved <path> placeholder.
• Preserve spaces, Unicode, and valid POSIX backslash filename characters while normalizing Windows path separators.
• Document the contract in both English and Japanese and add the bilingual changelog fragment changelog.d/unreleased/4899.fixed.md.

Root cause and impact

The prompt metadata described path as optional and prompts/get rendered a placeholder when it was absent. Path input also lacked prompt-specific validation. Clients could therefore receive a non-actionable template, and unsafe or platform-misclassified path text could be interpolated into the prompt. The prompt now fails early with typed errors and only renders a validated concrete indexed path.

Validation

• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -f net8.0 -p:UseSharedCompilation=false --no-restore --filter 'FullyQualifiedName~Prompts' — 18 passed
• dotnet test tests/CodeIndex.Tests/CodeIndex.Tests.csproj -c Release -f net9.0 -p:UseSharedCompilation=false --no-restore --filter 'FullyQualifiedName~Prompts' — 18 passed
• dotnet build CodeIndex.sln -c Release -p:UseSharedCompilation=false --no-restore — 0 warnings, 0 errors
• dotnet build src/CodeIndex/CodeIndex.csproj -c Debug -f net8.0 -p:UseSharedCompilation=false --no-restore — 0 warnings, 0 errors
• dotnet format CodeIndex.sln --verify-no-changes --no-restore
• dotnet run --project tools/CodeIndex.Changelog -- check — 74 fragments validated
• Manual stdio MCP verification: embedded newline rejected with -32602; POSIX src\..\file.cs preserved
• Final local index freshness/graph checks passed after updating 1,113 files and completing backfill-fold
• Full net8 suite before the final main refresh: 10,973 passed, 7 skipped
• Full net9 suite before the final main refresh: 10,497 passed, 420 skipped, with two unrelated load-sensitive lifecycle failures; both passed in isolated reruns and are tracked in Stabilize net9 MCP lifecycle tests under full-suite load #5001

Adversarial review

Two mandatory Codex adversarial-review rounds were completed. The review findings covered POSIX backslash preservation, control-character rejection, and platform-aware traversal validation; all reported findings were addressed with regression coverage.

Follow-up

• Stabilize net9 MCP lifecycle tests under full-suite load #5001

Fixes #4899