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/core/src/codex_delegate.rs

@@ -74,6 +74,8 @@ pub(crate) async fn run_codex_thread_interactive(
 ) -> Result<Codex, CodexErr> {
     let (tx_sub, rx_sub) = async_channel::bounded(SUBMISSION_CHANNEL_CAPACITY);
     let (tx_ops, rx_ops) = async_channel::bounded(SUBMISSION_CHANNEL_CAPACITY);
+    let conversation_history = initial_history.unwrap_or(InitialHistory::New);
+    let forked_from_thread_id = conversation_history.forked_from_id();
     let CodexSpawnOk { codex, .. } = Box::pin(Codex::spawn(CodexSpawnArgs {
         config,
         installation_id: parent_session.installation_id.clone(),
@@ -84,9 +86,10 @@ pub(crate) async fn run_codex_thread_interactive(
         plugins_manager: Arc::clone(&parent_session.services.plugins_manager),
         mcp_manager: Arc::clone(&parent_session.services.mcp_manager),
         extensions: Arc::clone(&parent_session.services.extensions),
-        conversation_history: initial_history.unwrap_or(InitialHistory::New),
+        conversation_history,
         session_source: SessionSource::SubAgent(subagent_source.clone()),
-        forked_from_thread_id: Some(parent_session.conversation_id),
+        forked_from_thread_id,
+        parent_thread_id: Some(parent_session.conversation_id),
         thread_source: Some(ThreadSource::Subagent),
         agent_control: parent_session.services.agent_control.clone(),
         dynamic_tools: Vec::new(),