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/app-server/tests/common/rollout.rs

@@ -1,4 +1,5 @@
 use anyhow::Result;
+use codex_protocol::SessionId;
 use codex_protocol::ThreadId;
 use codex_protocol::protocol::EventMsg;
 use codex_protocol::protocol::GitInfo;
@@ -127,11 +128,12 @@ pub fn create_fake_rollout_with_source(
         model_provider,
         git_info,
         source,
+        /*session_id*/ None,
         /*parent_thread_id*/ None,
     )
 }
 
-/// Create a minimal rollout file with an explicit session source and control parent.
+/// Create a minimal rollout file with an explicit root session and control parent.
 #[allow(clippy::too_many_arguments)]
 pub fn create_fake_parented_rollout_with_source(
     codex_home: &Path,
@@ -141,6 +143,7 @@ pub fn create_fake_parented_rollout_with_source(
     model_provider: Option<&str>,
     git_info: Option<GitInfo>,
     source: SessionSource,
+    session_id: SessionId,
     parent_thread_id: ThreadId,
 ) -> Result<String> {
     create_fake_rollout_with_source_and_parent_thread_id(
@@ -151,6 +154,7 @@ pub fn create_fake_parented_rollout_with_source(
         model_provider,
         git_info,
         source,
+        Some(session_id),
         Some(parent_thread_id),
     )
 }
@@ -164,11 +168,13 @@ fn create_fake_rollout_with_source_and_parent_thread_id(
     model_provider: Option<&str>,
     git_info: Option<GitInfo>,
     source: SessionSource,
+    session_id: Option<SessionId>,
     parent_thread_id: Option<ThreadId>,
 ) -> Result<String> {
     let uuid = Uuid::new_v4();
     let uuid_str = uuid.to_string();
     let conversation_id = ThreadId::from_string(&uuid_str)?;
+    let session_id = session_id.unwrap_or_else(|| conversation_id.into());
 
     let file_path = rollout_path(codex_home, filename_ts, &uuid_str);
     let dir = file_path
@@ -178,6 +184,7 @@ fn create_fake_rollout_with_source_and_parent_thread_id(
 
     // Build JSONL lines
     let meta = SessionMeta {
+        session_id,
         id: conversation_id,
         forked_from_id: None,
         parent_thread_id,
@@ -264,6 +271,7 @@ pub fn create_fake_rollout_with_text_elements(
 
     // Build JSONL lines
     let meta = SessionMeta {
+        session_id: conversation_id.into(),
         id: conversation_id,
         forked_from_id: None,
         parent_thread_id: None,