Add session/setTitle method

[daniel-agentee]

Summary

Adds session/setTitle to the Agent Client Protocol, giving clients a standard way to ask an agent to rename an existing session.

Request:  { sessionId: SessionId, title: String }
Response: {}

Title is plain UTF-8. Empty string remains valid at the protocol level so agents can decide whether that means "clear", "reset", or "use an empty title".

Why

Zed's first-party agent can already rename sessions, but ACP-backed sessions cannot, because there is no protocol method for the client to push a new title to the agent. The user-visible symptom is that the title editor in the agent panel appears for ACP threads but the rename cannot be persisted.

SessionInfoUpdate already exists in the reverse direction (agent -> client) and carries title -- this PR adds the missing client -> agent half of the round-trip.

How

• Adds SetSessionTitleRequest and SetSessionTitleResponse to the generated v1 and v2 agent schemas.
• Adds SESSION_SET_TITLE_METHOD_NAME = "session/setTitle" to the generated metadata.
• Wires v2-to-v1 and v1-to-v2 conversion for the new request/response while keeping removed v1 session/set_mode variants non-convertible to v2.
• Updates generated schema docs and JSON schema artifacts.
• Extends the schema generator's generated-type set for SetSessionTitle.

Tests

npm run generate
npm run check

npm run check covers clippy with all features, Prettier/cargo formatting checks, spellcheck, all-feature unit tests, generated-bin tests, and doctests.

Companion PRs

Companion codex-acp PRs:

• Handle session/setTitle and emit SessionInfoUpdate zed-industries/codex-acp#286 -- handle session/setTitle and emit SessionInfoUpdate.
• Add /rename builtin slash command zed-industries/codex-acp#287 -- add /rename <new title> as an ACP-client-friendly shortcut.

[ignatov]

@benbrandt @anna239, I believe it makes sense to decide on that PR on the next meeting

[daniel-agentee]

Rebased onto current main and regenerated schema/docs. Local validation is green: npm run generate and npm run check.

@codex review

[daniel-agentee]

Updated the branch to include sessionCapabilities.setTitle alongside the session/setTitle method. Regenerated schema/docs and reran npm run generate plus npm run check (clippy, format check, spellcheck, tests, doctests) successfully.

@codex review