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/app-server/src/bespoke_event_handling.rs

@@ -2170,6 +2170,7 @@ mod tests {
             thread_id,
             rollout_path: None,
             forked_from_id: None,
+            parent_thread_id: None,
             preview: "fallback preview".to_string(),
             name: Some("Rollback thread".to_string()),
             model_provider: "openai".to_string(),

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

@@ -4012,6 +4012,7 @@ pub(crate) fn thread_from_stored_thread(
         id: thread_id.clone(),
         session_id: thread_id,
         forked_from_id: thread.forked_from_id.map(|id| id.to_string()),
+        parent_thread_id: thread.parent_thread_id.map(|id| id.to_string()),
         preview: thread.preview,
         ephemeral: false,
         model_provider: if thread.model_provider.is_empty() {
@@ -4220,6 +4221,7 @@ fn build_thread_from_snapshot(
         id: thread_id.to_string(),
         session_id,
         forked_from_id: None,
+        parent_thread_id: config_snapshot.parent_thread_id.map(|id| id.to_string()),
         preview: String::new(),
         ephemeral: config_snapshot.ephemeral,
         model_provider: config_snapshot.model_provider_id.clone(),

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

@@ -395,6 +395,7 @@ mod thread_processor_behavior_tests {
             thread_id,
             rollout_path: Some(PathBuf::from("/tmp/thread.jsonl")),
             forked_from_id: None,
+            parent_thread_id: None,
             preview: "preview".to_string(),
             name: None,
             model_provider: "openai".to_string(),
@@ -681,6 +682,7 @@ mod thread_processor_behavior_tests {
                 },
             },
             session_source: SessionSource::Cli,
+            parent_thread_id: None,
             thread_source: None,
         };
 

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

@@ -172,6 +172,7 @@ mod tests {
             id: "thread-1".to_string(),
             session_id: "session-1".to_string(),
             forked_from_id: None,
+            parent_thread_id: None,
             preview: "preview".to_string(),
             ephemeral: false,
             model_provider: "mock_provider".to_string(),

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

@@ -305,6 +305,7 @@ pub(crate) fn summary_to_thread(
         id: thread_id.clone(),
         session_id: thread_id,
         forked_from_id: None,
+        parent_thread_id: None,
         preview,
         ephemeral: false,
         model_provider,

codex-rs/app-server/src/thread_status.rs

@@ -891,6 +891,7 @@ mod tests {
             id: thread_id.to_string(),
             session_id: thread_id.to_string(),
             forked_from_id: None,
+            parent_thread_id: None,
             preview: String::new(),
             ephemeral: false,
             model_provider: "mock-provider".to_string(),