Require a path for the summarize_file MCP prompt

[Widthdom]

Summary

Calling the summarize_file prompt without arguments returns an unresolved <path> placeholder instead of a useful default or typed validation error.

Audit IDs: G-30. Reproduced with locally built cdidx v1.40.3 at 404db09489c305f17d94f37026e00564d3ac527a.

Existing issue relationship

Closed #3121 is adjacent MCP prompt argument work.

Expected behavior and PR scope

Make path required and reject missing/blank values with InvalidParams, or implement a genuinely documented contextual default. Do not emit unresolved template text. Keep this prompt-specific.

Implementation guide for Codex 5.6 sol xhigh

• Align prompts/list argument metadata, prompts/get validation, and template rendering.
• Normalize/validate workspace-relative path and redaction/security policy before rendering.
• Cover missing/null/blank, valid/invalid/outside-workspace paths, spaces/Unicode, and protocol errors.
• If contextual default is chosen, define the context source and deterministic fallback explicitly.
• Add focused regression/contract tests, run the full net8.0 and net9.0 suite, and add the required bilingual changelog.d/unreleased/ fragment.

Acceptance criteria

• No response contains unresolved <path>.
• Typed clients see path as required (or a documented default).
• Invalid paths fail before prompt construction.

[Widthdom]

Work started on branch fix-issue4899.