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/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,