[FEATURE]: Add server-side transcript search to the V2 message API

Feature hasn't been suggested before

• I searched open and closed issues. Existing requests cover TUI/Desktop search UI or cross-session discovery, but not a public V2 transcript-search contract for bounded clients.

Describe the enhancement you want to request

The V2 message-list operation (v2.message.list) accepts only sessionID, limit, order, and cursor. The search parameter on v2.session.list filters session titles, not message content.

Clients that keep only a bounded transcript window must therefore traverse every message page to find text outside the resident window. CodeNomad currently keeps 200 messages resident and performs this exhaustive traversal for full-session search. Search cost grows with the complete transcript, and a result cannot identify a directly navigable target or prove that all searched pages belong to one transcript revision.

Please add server-side text search scoped to one session. A minimal contract should:

• search the complete projected user and assistant text for that session;
• paginate matches;
• return the matching messageID, partID when applicable, and a compact excerpt;
• return a target that can be opened without walking every preceding message cursor;
• identify the transcript revision used for the results.

A case-insensitive literal search is sufficient initially. Fuzzy or semantic search and a new indexing subsystem are not required.

This is an API request rather than another UI request. Existing related UI issues include #4714, #19143, #38973, and #41354. The navigation target should compose with #44660, and result consistency should compose with #43766.

Acceptance criteria

• A client can search the full projected transcript without downloading every message page.
• Results are paginated in a documented stable order while the transcript revision is unchanged.
• Each result identifies the exact message/part and includes an excerpt.
• A client can navigate directly to the matching transcript area.
• Results expose the revision needed to detect compaction, revert, append, or content-update races.
• Existing session-title search and message cursor pagination remain backward compatible.