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/analytics/src/analytics_client_tests.rs

@@ -160,11 +160,13 @@ fn sample_thread_with_metadata(
     ephemeral: bool,
     source: AppServerSessionSource,
     thread_source: Option<AppServerThreadSource>,
+    parent_thread_id: Option<String>,
 ) -> Thread {
     Thread {
         id: thread_id.to_string(),
         session_id: format!("session-{thread_id}"),
         forked_from_id: None,
+        parent_thread_id,
         preview: "first prompt".to_string(),
         ephemeral,
         model_provider: "openai".to_string(),
@@ -195,6 +197,7 @@ fn sample_thread_start_response(
             ephemeral,
             AppServerSessionSource::Exec,
             Some(AppServerThreadSource::User),
+            /*parent_thread_id*/ None,
         ),
         model: model.to_string(),
         model_provider: "openai".to_string(),
@@ -240,6 +243,7 @@ fn sample_thread_resume_response(
         model,
         AppServerSessionSource::Exec,
         Some(AppServerThreadSource::User),
+        /*parent_thread_id*/ None,
     )
 }
 
@@ -249,9 +253,16 @@ fn sample_thread_resume_response_with_source(
     model: &str,
     source: AppServerSessionSource,
     thread_source: Option<AppServerThreadSource>,
+    parent_thread_id: Option<String>,
 ) -> ClientResponsePayload {
     ClientResponsePayload::ThreadResume(ThreadResumeResponse {
-        thread: sample_thread_with_metadata(thread_id, ephemeral, source, thread_source),
+        thread: sample_thread_with_metadata(
+            thread_id,
+            ephemeral,
+            source,
+            thread_source,
+            parent_thread_id,
+        ),
         model: model.to_string(),
         model_provider: "openai".to_string(),
         service_tier: None,
@@ -1755,6 +1766,7 @@ async fn compaction_event_ingests_custom_fact() {
                         agent_role: None,
                     }),
                     Some(AppServerThreadSource::Subagent),
+                    Some(parent_thread_id.to_string()),
                 )),
             },
             &mut events,
@@ -2456,7 +2468,7 @@ fn subagent_thread_started_thread_spawn_serializes_parent_thread_id() {
         SubAgentThreadStartedInput {
             session_id: "session-root".to_string(),
             thread_id: "thread-spawn".to_string(),
-            parent_thread_id: None,
+            parent_thread_id: Some(parent_thread_id.to_string()),
             product_client_id: "codex-tui".to_string(),
             client_name: "codex-tui".to_string(),
             client_version: "1.0.0".to_string(),
@@ -2534,11 +2546,14 @@ fn subagent_thread_started_other_serializes_expected_shape() {
 
 #[test]
 fn subagent_thread_started_other_serializes_explicit_parent_thread_id() {
+    let parent_thread_id =
+        codex_protocol::ThreadId::from_string("33333333-3333-4333-8333-333333333333")
+            .expect("valid thread id");
     let event = TrackEventRequest::ThreadInitialized(subagent_thread_started_event_request(
         SubAgentThreadStartedInput {
             session_id: "session-root".to_string(),
             thread_id: "thread-guardian".to_string(),
-            parent_thread_id: Some("parent-thread-guardian".to_string()),
+            parent_thread_id: Some(parent_thread_id.to_string()),
             product_client_id: "codex-tui".to_string(),
             client_name: "codex-tui".to_string(),
             client_version: "1.0.0".to_string(),
@@ -2553,7 +2568,7 @@ fn subagent_thread_started_other_serializes_explicit_parent_thread_id() {
     assert_eq!(payload["event_params"]["subagent_source"], "guardian");
     assert_eq!(
         payload["event_params"]["parent_thread_id"],
-        "parent-thread-guardian"
+        "33333333-3333-4333-8333-333333333333"
     );
 }
 
@@ -2642,7 +2657,7 @@ async fn subagent_thread_started_inherits_parent_connection_for_new_thread() {
                 SubAgentThreadStartedInput {
                     session_id: "session-root".to_string(),
                     thread_id: "thread-review".to_string(),
-                    parent_thread_id: None,
+                    parent_thread_id: Some(parent_thread_id.to_string()),
                     product_client_id: "parent-client".to_string(),
                     client_name: "parent-client".to_string(),
                     client_version: "1.0.0".to_string(),