Persist session IDs across thread resume (#29327)

## Summary

A cold-resumed subagent kept its durable thread ID but could receive a
new session ID, splitting one agent tree across multiple sessions after
a restart.

Persist the root session ID in every rollout `SessionMeta`, carry it
through thread creation, and restore it before initializing the resumed
`Session` and `AgentControl`.

## Behavior

For a nested agent tree:

```text
root session R
  parent thread P
    child thread C
```

The child rollout stores:

```text
session_id:       R
parent_thread_id: P
id:               C
```

After a cold resume, the child still belongs to root session `R` while
its immediate parent remains `P`. The integration coverage uses distinct
values for all three IDs so it catches restoring the session from
`parent_thread_id`.

## Legacy rollouts

Previous rollouts have `id` but no `session_id`. `SessionMetaLine`
deserialization treats a missing `session_id` as `id`, keeping those
files readable, listable, and resumable. When a legacy subagent is
resumed through its root, that synthesized child ID no longer overrides
the inherited root-scoped `AgentControl`. New rollouts always persist
the explicit root session ID.

codex-rs/thread-store/src/in_memory.rs

@@ -110,6 +110,7 @@ mod tests {
         ] {
             store
                 .create_thread(CreateThreadParams {
+                    session_id: thread_id.into(),
                     thread_id,
                     extra_config: None,
                     forked_from_id: None,
@@ -231,6 +232,7 @@ impl InMemoryThreadStore {
         let mut state = self.state.lock().await;
         state.calls.create_thread += 1;
         let session_meta = SessionMeta {
+            session_id: params.session_id,
             id: params.thread_id,
             forked_from_id: params.forked_from_id,
             parent_thread_id: params.parent_thread_id,