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/core/src/session/tests.rs

@@ -1900,6 +1900,7 @@ fn session_meta_item(
 ) -> RolloutItem {
     RolloutItem::SessionMeta(SessionMetaLine {
         meta: SessionMeta {
+            session_id: thread_id.into(),
             id: thread_id,
             multi_agent_version,
             ..SessionMeta::default()
@@ -3750,6 +3751,7 @@ async fn attach_thread_persistence(session: &mut Session) -> PathBuf {
     let live_thread = LiveThread::create(
         Arc::clone(&session.services.thread_store),
         CreateThreadParams {
+            session_id: session.session_id(),
             thread_id: session.thread_id,
             extra_config: None,
             forked_from_id: None,
@@ -5453,7 +5455,7 @@ async fn resumed_root_session_uses_thread_id_as_session_id() {
 }
 
 #[tokio::test]
-async fn resumed_subagent_session_keeps_inherited_session_id() {
+async fn resumed_subagent_session_restores_persisted_session_id() {
     let parent_thread_id = ThreadId::new();
     let parent_session_id = SessionId::from(parent_thread_id);
     let thread_id = ThreadId::new();
@@ -5467,11 +5469,19 @@ async fn resumed_subagent_session_keeps_inherited_session_id() {
     let (session, rx_event) = make_session_with_history_source_and_agent_control_and_rx(
         InitialHistory::Resumed(ResumedHistory {
             conversation_id: thread_id,
-            history: Vec::new(),
+            history: vec![RolloutItem::SessionMeta(SessionMetaLine {
+                meta: SessionMeta {
+                    session_id: parent_session_id,
+                    id: thread_id,
+                    source: session_source.clone(),
+                    ..SessionMeta::default()
+                },
+                git: None,
+            })],
             rollout_path: None,
         }),
         session_source,
-        AgentControl::default().with_session_id(parent_session_id, /*max_threads*/ usize::MAX),
+        AgentControl::default(),
     )
     .await
     .expect("resume should succeed");
@@ -6590,6 +6600,7 @@ async fn shutdown_complete_does_not_append_to_thread_store_after_shutdown() {
     let live_thread = LiveThread::create(
         Arc::clone(&thread_store),
         CreateThreadParams {
+            session_id: session.session_id(),
             thread_id: session.thread_id,
             extra_config: None,
             forked_from_id: None,