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/client.rs

@@ -173,6 +173,7 @@ struct ModelClientState {
     provider: SharedModelProvider,
     auth_env_telemetry: AuthEnvTelemetry,
     session_source: SessionSource,
+    parent_thread_id: Option<ThreadId>,
     model_verbosity: Option<VerbosityConfig>,
     enable_request_compression: bool,
     include_timing_metrics: bool,
@@ -321,6 +322,7 @@ impl ModelClient {
         installation_id: String,
         provider_info: ModelProviderInfo,
         session_source: SessionSource,
+        parent_thread_id: Option<ThreadId>,
         model_verbosity: Option<VerbosityConfig>,
         enable_request_compression: bool,
         include_timing_metrics: bool,
@@ -344,6 +346,7 @@ impl ModelClient {
                 provider: model_provider,
                 auth_env_telemetry,
                 session_source,
+                parent_thread_id,
                 model_verbosity,
                 enable_request_compression,
                 include_timing_metrics,
@@ -637,7 +640,7 @@ impl ModelClient {
 
     fn build_responses_identity_headers(&self) -> ApiHeaderMap {
         let mut extra_headers = self.build_subagent_headers();
-        if let Some(parent_thread_id) = parent_thread_id_header_value(&self.state.session_source)
+        if let Some(parent_thread_id) = parent_thread_id_header_value(self.state.parent_thread_id)
             && let Ok(val) = HeaderValue::from_str(&parent_thread_id)
         {
             extra_headers.insert(X_CODEX_PARENT_THREAD_ID_HEADER, val);
@@ -664,7 +667,7 @@ impl ModelClient {
         if let Some(subagent) = subagent_header_value(&self.state.session_source) {
             client_metadata.insert(X_OPENAI_SUBAGENT_HEADER.to_string(), subagent);
         }
-        if let Some(parent_thread_id) = parent_thread_id_header_value(&self.state.session_source) {
+        if let Some(parent_thread_id) = parent_thread_id_header_value(self.state.parent_thread_id) {
             client_metadata.insert(
                 X_CODEX_PARENT_THREAD_ID_HEADER.to_string(),
                 parent_thread_id,
@@ -1733,10 +1736,8 @@ fn subagent_header_value(session_source: &SessionSource) -> Option<String> {
     }
 }
 
-fn parent_thread_id_header_value(session_source: &SessionSource) -> Option<String> {
-    session_source
-        .parent_thread_id()
-        .map(|parent_thread_id| parent_thread_id.to_string())
+fn parent_thread_id_header_value(parent_thread_id: Option<ThreadId>) -> Option<String> {
+    parent_thread_id.map(|parent_thread_id| parent_thread_id.to_string())
 }
 
 const RESPONSE_STREAM_CHANNEL_CAPACITY: usize = 1600;