store and expose parent_thread_id on Threads (#25113)

## Why

This PR
https://github.com/openai/codex/pull/24161#discussion_r3325692763
revealed a subagent data modeling issue, where we overloaded
`forked_from_id` to also mean `parent_thread_id`. That's incorrect since
guardian and review subagents can be a subagent and NOT fork the main
thread's history.

The solution here is to explicitly store a new `parent_thread_id` on
`SessionMeta`, alongside `forked_from_id` which already exists. While
we're at it, also expose it in the app-server protocol on the `Thread`
object.

A thread->subagent relationship and a fork of thread history are
orthogonal concepts.

## What Changed

- Added top-level `parent_thread_id` persistence on `SessionMeta` and
runtime/session plumbing through `SessionConfiguredEvent`,
`CodexSpawnArgs`, `SessionConfiguration`, `ThreadConfigSnapshot`,
`TurnContext`, and `ModelClient`.
- Made turn metadata, request headers, analytics, and subagent-start
events read the separate runtime/top-level parent field instead of
deriving general parent lineage from `SessionSource` or
`forked_from_thread_id`.
- Passed parent lineage separately at delegated subagent, review,
guardian, agent-job, and multi-agent spawn construction sites;
copied-history fork lineage remains derived only from `InitialHistory`.
- Persisted and exposed parent lineage through rollout/thread-store
projections and app-server v2 `Thread.parentThreadId`.
- Updated app-server README text and regenerated app-server schema
fixtures for the additive `parentThreadId` response field.

codex-rs/exec/src/event_processor_with_human_output_tests.rs

@@ -211,6 +211,7 @@ async fn config_summary_entries_include_runtime_workspace_roots() {
         session_id: SessionId::new(),
         thread_id: ThreadId::new(),
         forked_from_id: None,
+        parent_thread_id: None,
         thread_source: None,
         thread_name: None,
         model: "gpt-5.4".to_string(),

codex-rs/exec/src/lib.rs

@@ -1070,6 +1070,7 @@ fn session_configured_from_thread_start_response(
     session_configured_from_thread_response(
         &response.thread.session_id,
         &response.thread.id,
+        response.thread.parent_thread_id.as_deref(),
         response.thread.thread_source.map(Into::into),
         response.thread.name.clone(),
         response.thread.path.clone(),
@@ -1092,6 +1093,7 @@ fn session_configured_from_thread_resume_response(
     session_configured_from_thread_response(
         &response.thread.session_id,
         &response.thread.id,
+        response.thread.parent_thread_id.as_deref(),
         response.thread.thread_source.map(Into::into),
         response.thread.name.clone(),
         response.thread.path.clone(),
@@ -1123,6 +1125,7 @@ fn review_target_to_api(target: ReviewTarget) -> ApiReviewTarget {
 fn session_configured_from_thread_response(
     session_id: &str,
     thread_id: &str,
+    parent_thread_id: Option<&str>,
     thread_source: Option<codex_protocol::protocol::ThreadSource>,
     thread_name: Option<String>,
     rollout_path: Option<PathBuf>,
@@ -1140,11 +1143,16 @@ fn session_configured_from_thread_response(
         .map_err(|err| format!("session id `{session_id}` is invalid: {err}"))?;
     let thread_id = ThreadId::from_string(thread_id)
         .map_err(|err| format!("thread id `{thread_id}` is invalid: {err}"))?;
+    let parent_thread_id = parent_thread_id
+        .map(ThreadId::from_string)
+        .transpose()
+        .map_err(|err| format!("parent thread id is invalid: {err}"))?;
 
     Ok(SessionConfiguredEvent {
         session_id,
         thread_id,
         forked_from_id: None,
+        parent_thread_id,
         thread_source,
         thread_name,
         model,

codex-rs/exec/src/lib_tests.rs

@@ -307,6 +307,7 @@ fn turn_items_for_thread_returns_matching_turn_items() {
         id: "thread-1".to_string(),
         session_id: "thread-1".to_string(),
         forked_from_id: None,
+        parent_thread_id: None,
         preview: String::new(),
         ephemeral: false,
         model_provider: "openai".to_string(),
@@ -585,12 +586,33 @@ async fn session_configured_from_thread_response_preserves_thread_source() {
     );
 }
 
+#[tokio::test]
+async fn session_configured_from_thread_response_preserves_parent_thread_id() {
+    let codex_home = tempdir().expect("create temp codex home");
+    let cwd = tempdir().expect("create temp cwd");
+    let config = ConfigBuilder::default()
+        .codex_home(codex_home.path().to_path_buf())
+        .fallback_cwd(Some(cwd.path().to_path_buf()))
+        .build()
+        .await
+        .expect("build config");
+    let parent_thread_id = ThreadId::new();
+    let mut response = sample_thread_start_response();
+    response.thread.parent_thread_id = Some(parent_thread_id.to_string());
+
+    let event = session_configured_from_thread_start_response(&response, &config)
+        .expect("build bootstrap session configured event");
+
+    assert_eq!(event.parent_thread_id, Some(parent_thread_id));
+}
+
 fn sample_thread_start_response() -> ThreadStartResponse {
     ThreadStartResponse {
         thread: codex_app_server_protocol::Thread {
             id: "67e55044-10b1-426f-9247-bb680e5fe0c8".to_string(),
             session_id: "67e55044-10b1-426f-9247-bb680e5fe0c7".to_string(),
             forked_from_id: None,
+            parent_thread_id: None,
             preview: String::new(),
             ephemeral: false,
             model_provider: "openai".to_string(),