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/session/session.rs

@@ -97,6 +97,8 @@ pub(crate) struct SessionConfiguration {
     pub(super) session_source: SessionSource,
     /// Immediate history source copied into this thread, when this thread was forked.
     pub(super) forked_from_thread_id: Option<ThreadId>,
+    /// Immediate control/spawn parent for this thread, when it has one.
+    pub(super) parent_thread_id: Option<ThreadId>,
     /// Optional analytics source classification for this thread.
     pub(super) thread_source: Option<ThreadSource>,
     pub(super) dynamic_tools: Vec<DynamicToolSpec>,
@@ -187,6 +189,7 @@ impl SessionConfiguration {
             personality: self.personality,
             collaboration_mode: self.collaboration_mode.clone(),
             session_source: self.session_source.clone(),
+            parent_thread_id: self.parent_thread_id,
             thread_source: self.thread_source,
         }
     }
@@ -511,6 +514,10 @@ impl Session {
             .forked_from_thread_id
             .or_else(|| initial_history.forked_from_id());
         session_configuration.forked_from_thread_id = forked_from_id;
+        let parent_thread_id = session_configuration
+            .parent_thread_id
+            .or_else(|| initial_history.get_resumed_parent_thread_id());
+        session_configuration.parent_thread_id = parent_thread_id;
 
         let event_persistence_mode = if session_configuration.persist_extended_history {
             ThreadEventPersistenceMode::Extended
@@ -550,6 +557,7 @@ impl Session {
                             CreateThreadParams {
                                 thread_id,
                                 forked_from_id,
+                                parent_thread_id,
                                 source: session_source,
                                 thread_source: session_configuration.thread_source,
                                 base_instructions: BaseInstructions {
@@ -1031,6 +1039,7 @@ impl Session {
                     installation_id.clone(),
                     session_configuration.provider.clone(),
                     session_configuration.session_source.clone(),
+                    session_configuration.parent_thread_id,
                     config.model_verbosity,
                     config.features.enabled(Feature::EnableRequestCompression),
                     config.features.enabled(Feature::RuntimeMetrics),
@@ -1040,7 +1049,7 @@ impl Session {
                 .with_prompt_cache_key_override(
                     crate::guardian::prompt_cache_key_override_for_review_session(
                         &session_configuration.session_source,
-                        session_configuration.forked_from_thread_id,
+                        session_configuration.parent_thread_id,
                     ),
                 ),
                 code_mode_service: crate::tools::code_mode::CodeModeService::new(),
@@ -1083,6 +1092,7 @@ impl Session {
                     session_id,
                     thread_id,
                     forked_from_id,
+                    parent_thread_id,
                     thread_source: session_configuration.thread_source,
                     thread_name: session_configuration.thread_name.clone(),
                     model: session_configuration.collaboration_mode.model().to_string(),