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/src/request_processors/external_agent_session_import.rs

@@ -200,6 +200,7 @@ impl ExternalAgentSessionImporter {
         };
         let now = Utc::now();
         let create_params = CreateThreadParams {
+            session_id: thread_id.into(),
             thread_id,
             extra_config: None,
             forked_from_id: None,

codex-rs/app-server/src/request_processors/thread_processor_tests.rs

@@ -1004,6 +1004,7 @@ mod thread_processor_behavior_tests {
         let timestamp = "2025-09-05T16:53:11.850Z".to_string();
 
         let session_meta = SessionMeta {
+            session_id: conversation_id.into(),
             id: conversation_id,
             timestamp: timestamp.clone(),
             model_provider: None,
@@ -1060,6 +1061,7 @@ mod thread_processor_behavior_tests {
         let timestamp = "2025-09-05T16:53:11.850Z".to_string();
 
         let session_meta = SessionMeta {
+            session_id: parent_thread_id.into(),
             id: conversation_id,
             timestamp: timestamp.clone(),
             source: SessionSource::SubAgent(SubAgentSource::ThreadSpawn {
@@ -1110,6 +1112,7 @@ mod thread_processor_behavior_tests {
         let timestamp = "2025-09-05T16:53:11.850Z".to_string();
 
         let session_meta = SessionMeta {
+            session_id: conversation_id.into(),
             id: conversation_id,
             forked_from_id: Some(forked_from_id),
             timestamp: timestamp.clone(),

codex-rs/app-server/src/request_processors/thread_summary_tests.rs

@@ -13,6 +13,7 @@ fn extract_conversation_summary_prefers_plain_user_messages() -> Result<()> {
 
     let head = vec![
         json!({
+            "session_id": conversation_id.to_string(),
             "id": conversation_id.to_string(),
             "timestamp": timestamp,
             "cwd": "/",