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/local/create_thread.rs

@@ -36,6 +36,7 @@ pub(super) async fn create_thread(
             params.base_instructions,
             params.dynamic_tools,
         )
+        .with_session_id(params.session_id)
         .with_multi_agent_version(params.multi_agent_version),
     )
     .await

codex-rs/thread-store/src/local/mod.rs

@@ -1123,6 +1123,7 @@ mod tests {
 
     fn create_thread_params(thread_id: ThreadId) -> CreateThreadParams {
         CreateThreadParams {
+            session_id: thread_id.into(),
             thread_id,
             extra_config: None,
             forked_from_id: None,

codex-rs/thread-store/src/local/read_thread.rs

@@ -821,6 +821,7 @@ mod tests {
             "timestamp": "2025-01-03T12:00:00Z",
             "type": "session_meta",
             "payload": {
+                "session_id": uuid,
                 "id": uuid,
                 "timestamp": "2025-01-03T12:00:00Z",
                 "cwd": rollout_cwd,
@@ -926,6 +927,7 @@ mod tests {
             "timestamp": "2025-01-03T12-00-00",
             "type": "session_meta",
             "payload": {
+                "session_id": uuid,
                 "id": uuid,
                 "timestamp": "2025-01-03T12-00-00",
                 "cwd": home.path(),
@@ -1091,6 +1093,7 @@ mod tests {
             "timestamp": "2025-01-03T12:00:00Z",
             "type": "session_meta",
             "payload": {
+                "session_id": uuid,
                 "id": uuid,
                 "timestamp": "2025-01-03T12:00:00Z",
                 "cwd": home.path(),

codex-rs/thread-store/src/local/test_support.rs

@@ -77,6 +77,7 @@ pub(super) fn write_session_file_with_fork(
         "timestamp": ts,
         "type": "session_meta",
         "payload": {
+            "session_id": uuid,
             "id": uuid,
             "forked_from_id": forked_from_id,
             "timestamp": ts,