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 conversion for the new request/response.
• Updates generated schema docs and JSON schema artifacts.
• Extends the schema generator's generated-type set for SetSessionTitle.

Tests

cargo test --workspace --locked
cargo build --locked

Companion PRs

Once this lands I'll open follow-ups for the agent (codex-acp / claude-agent-acp) and client (Zed) sides so the title-editor flow works end-to-end over ACP.