## Why
`thread/start` and `thread/resume` already return `sessionId`, but
`thread/fork` only returned the new thread. That left clients to infer
the forked thread's session identity from `thread.id`, which kept the
new `session_id` / `thread_id` split implicit at one lifecycle boundary.
Follow-up to #20437.
## What changed
- Add `sessionId` to `ThreadForkResponse`.
- Populate it from the forked session configuration.
- Regenerate the v2 JSON/TypeScript schema fixtures and update the
app-server docs/example.
- Extend the fork integration test to assert the returned `sessionId`.
## Verification
- Added coverage in `thread_fork_creates_new_thread_and_emits_started`
for the new response field.
06e5dfa4dd