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/tests/suite/client.rs

@@ -516,6 +516,7 @@ async fn resume_includes_initial_messages_and_sends_prior_items() {
             "timestamp": "2024-01-01T00:00:00.000Z",
             "type": "session_meta",
             "payload": {
+                "session_id": convo_id,
                 "id": convo_id,
                 "timestamp": "2024-01-01T00:00:00Z",
                 "instructions": "be nice",
@@ -726,12 +727,14 @@ async fn resume_replays_legacy_js_repl_image_rollout_shapes() {
         metadata: None,
     };
     let legacy_image_url = "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR4nGP4z8DwHwAFAAH/iZk9HQAAAABJRU5ErkJggg==";
+    let thread_id = ThreadId::default();
     let rollout = vec![
         RolloutLine {
             timestamp: "2024-01-01T00:00:00.000Z".to_string(),
             item: RolloutItem::SessionMeta(SessionMetaLine {
                 meta: SessionMeta {
-                    id: ThreadId::default(),
+                    session_id: thread_id.into(),
+                    id: thread_id,
                     parent_thread_id: None,
                     timestamp: "2024-01-01T00:00:00Z".to_string(),
                     cwd: ".".into(),
@@ -860,12 +863,14 @@ async fn resume_replays_image_tool_outputs_with_detail() {
     let image_url = "data:image/webp;base64,UklGRiIAAABXRUJQVlA4IBYAAAAwAQCdASoBAAEAAUAmJaACdLoB+AADsAD+8ut//NgVzXPv9//S4P0uD9Lg/9KQAAA=";
     let function_call_id = "view-image-call";
     let custom_call_id = "js-repl-call";
+    let thread_id = ThreadId::default();
     let rollout = vec![
         RolloutLine {
             timestamp: "2024-01-01T00:00:00.000Z".to_string(),
             item: RolloutItem::SessionMeta(SessionMetaLine {
                 meta: SessionMeta {
-                    id: ThreadId::default(),
+                    session_id: thread_id.into(),
+                    id: thread_id,
                     parent_thread_id: None,
                     timestamp: "2024-01-01T00:00:00Z".to_string(),
                     cwd: ".".into(),

codex-rs/core/tests/suite/personality_migration.rs

@@ -59,6 +59,7 @@ async fn write_rollout_with_user_event(dir: &Path, thread_id: ThreadId) -> io::R
 
     let session_meta = SessionMetaLine {
         meta: SessionMeta {
+            session_id: thread_id.into(),
             id: thread_id,
             forked_from_id: None,
             parent_thread_id: None,
@@ -109,6 +110,7 @@ async fn write_rollout_with_meta_only(dir: &Path, thread_id: ThreadId) -> io::Re
 
     let session_meta = SessionMetaLine {
         meta: SessionMeta {
+            session_id: thread_id.into(),
             id: thread_id,
             forked_from_id: None,
             parent_thread_id: None,

codex-rs/core/tests/suite/review.rs

@@ -511,6 +511,7 @@ async fn review_input_isolated_from_parent_history() {
             "timestamp": "2024-01-01T00:00:00.000Z",
             "type": "session_meta",
             "payload": {
+                "session_id": convo_id,
                 "id": convo_id,
                 "timestamp": "2024-01-01T00:00:00Z",
                 "cwd": ".",

codex-rs/core/tests/suite/rollout_list_find.rs

@@ -42,6 +42,7 @@ fn write_minimal_rollout_with_id_at_path(file: &Path, id: Uuid) {
             "timestamp": "2024-01-01T00:00:00.000Z",
             "type": "session_meta",
             "payload": {
+                "session_id": id,
                 "id": id,
                 "timestamp": "2024-01-01T00:00:00Z",
                 "cwd": ".",

codex-rs/core/tests/suite/sqlite_state.rs

@@ -356,6 +356,7 @@ async fn backfill_scans_existing_rollouts() -> Result<()> {
             fs::create_dir_all(parent).expect("should create rollout directory");
             let session_meta_line = SessionMetaLine {
                 meta: SessionMeta {
+                    session_id: thread_id.into(),
                     id: thread_id,
                     forked_from_id: None,
                     parent_thread_id: None,