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/tui/src/app/loaded_threads.rs

@@ -120,6 +120,7 @@ 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: "openai".to_string(),

codex-rs/tui/src/app/tests.rs

@@ -2767,6 +2767,7 @@ async fn inactive_thread_started_notification_initializes_replay_session() -> Re
                 id: agent_thread_id.to_string(),
                 session_id: agent_thread_id.to_string(),
                 forked_from_id: None,
+                parent_thread_id: None,
                 preview: "agent thread".to_string(),
                 ephemeral: false,
                 model_provider: "agent-provider".to_string(),
@@ -2856,6 +2857,7 @@ async fn inactive_thread_started_notification_preserves_primary_model_when_path_
                 id: agent_thread_id.to_string(),
                 session_id: agent_thread_id.to_string(),
                 forked_from_id: None,
+                parent_thread_id: None,
                 preview: "agent thread".to_string(),
                 ephemeral: false,
                 model_provider: "agent-provider".to_string(),
@@ -2914,6 +2916,7 @@ async fn thread_read_session_state_does_not_reuse_primary_permission_profile() {
         id: read_thread_id.to_string(),
         session_id: read_thread_id.to_string(),
         forked_from_id: None,
+        parent_thread_id: None,
         preview: "read thread".to_string(),
         ephemeral: false,
         model_provider: "read-provider".to_string(),
@@ -5011,6 +5014,7 @@ async fn thread_rollback_response_discards_queued_active_thread_events() {
                 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: "openai".to_string(),

codex-rs/tui/src/app/thread_session_state.rs

@@ -411,6 +411,7 @@ mod tests {
             id: read_thread_id.to_string(),
             session_id: read_thread_id.to_string(),
             forked_from_id: None,
+            parent_thread_id: None,
             preview: "read thread".to_string(),
             ephemeral: false,
             model_provider: "read-provider".to_string(),