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/app-server-protocol/src/protocol/v2/thread.rs

@@ -419,6 +419,9 @@ pub struct ThreadForkParams {
 #[serde(rename_all = "camelCase")]
 #[ts(export_to = "v2/")]
 pub struct ThreadForkResponse {
+    /// Session id shared by threads that belong to the same session tree.
+    #[serde(default)]
+    pub session_id: String,
     pub thread: Thread,
     pub model: String,
     pub model_provider: String,