feat: return session ID from thread/fork (#21332)

## 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.

codex-rs/analytics/src/client_tests.rs

@@ -138,6 +138,7 @@ fn sample_thread_resume_response() -> ClientResponsePayload {
 
 fn sample_thread_fork_response() -> ClientResponsePayload {
     ClientResponsePayload::ThreadFork(ThreadForkResponse {
+        session_id: "session-3".to_string(),
         thread: sample_thread("thread-3"),
         model: "gpt-5".to_string(),
         model_provider: "openai".to_string(),