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/thread_manager.rs

@@ -187,6 +187,7 @@ pub(crate) struct ResumeThreadWithHistoryOptions {
     pub(crate) initial_history: InitialHistory,
     pub(crate) agent_control: AgentControl,
     pub(crate) session_source: SessionSource,
+    pub(crate) parent_thread_id: Option<ThreadId>,
     pub(crate) inherited_shell_snapshot: Option<Arc<ShellSnapshot>>,
     pub(crate) inherited_exec_policy: Option<Arc<crate::exec_policy::ExecPolicyManager>>,
 }
@@ -601,6 +602,7 @@ impl ThreadManager {
             Arc::clone(&self.state.auth_manager),
             self.agent_control(),
             session_source,
+            /*parent_thread_id*/ None,
             forked_from_thread_id,
             thread_source,
             options.dynamic_tools,
@@ -685,6 +687,7 @@ impl ThreadManager {
             auth_manager,
             self.agent_control(),
             session_source,
+            /*parent_thread_id*/ None,
             /*forked_from_thread_id*/ None,
             thread_source,
             Vec::new(),
@@ -713,6 +716,7 @@ impl ThreadManager {
             InitialHistory::New,
             Arc::clone(&self.state.auth_manager),
             self.agent_control(),
+            /*parent_thread_id*/ None,
             /*forked_from_thread_id*/ None,
             /*thread_source*/ None,
             Vec::new(),
@@ -746,6 +750,7 @@ impl ThreadManager {
             auth_manager,
             self.agent_control(),
             session_source,
+            /*parent_thread_id*/ None,
             /*forked_from_thread_id*/ None,
             thread_source,
             Vec::new(),
@@ -916,6 +921,7 @@ impl ThreadManager {
             history,
             Arc::clone(&self.state.auth_manager),
             self.agent_control(),
+            /*parent_thread_id*/ None,
             forked_from_thread_id,
             thread_source,
             Vec::new(),
@@ -1039,6 +1045,7 @@ impl ThreadManagerState {
             config,
             agent_control,
             self.session_source.clone(),
+            /*parent_thread_id*/ None,
             /*forked_from_thread_id*/ None,
             /*thread_source*/ None,
             /*persist_extended_history*/ false,
@@ -1056,6 +1063,7 @@ impl ThreadManagerState {
         config: Config,
         agent_control: AgentControl,
         session_source: SessionSource,
+        parent_thread_id: Option<ThreadId>,
         forked_from_thread_id: Option<ThreadId>,
         thread_source: Option<ThreadSource>,
         persist_extended_history: bool,
@@ -1073,6 +1081,7 @@ impl ThreadManagerState {
             Arc::clone(&self.auth_manager),
             agent_control,
             session_source,
+            parent_thread_id,
             forked_from_thread_id,
             thread_source,
             Vec::new(),
@@ -1096,6 +1105,7 @@ impl ThreadManagerState {
             initial_history,
             agent_control,
             session_source,
+            parent_thread_id,
             inherited_shell_snapshot,
             inherited_exec_policy,
         } = options;
@@ -1108,6 +1118,7 @@ impl ThreadManagerState {
             Arc::clone(&self.auth_manager),
             agent_control,
             session_source,
+            parent_thread_id,
             /*forked_from_thread_id*/ None,
             thread_source,
             Vec::new(),
@@ -1130,6 +1141,7 @@ impl ThreadManagerState {
         agent_control: AgentControl,
         session_source: SessionSource,
         thread_source: Option<ThreadSource>,
+        parent_thread_id: Option<ThreadId>,
         forked_from_thread_id: Option<ThreadId>,
         persist_extended_history: bool,
         inherited_shell_snapshot: Option<Arc<ShellSnapshot>>,
@@ -1145,6 +1157,7 @@ impl ThreadManagerState {
             Arc::clone(&self.auth_manager),
             agent_control,
             session_source,
+            parent_thread_id,
             forked_from_thread_id,
             thread_source,
             Vec::new(),
@@ -1167,6 +1180,7 @@ impl ThreadManagerState {
         initial_history: InitialHistory,
         auth_manager: Arc<AuthManager>,
         agent_control: AgentControl,
+        parent_thread_id: Option<ThreadId>,
         forked_from_thread_id: Option<ThreadId>,
         thread_source: Option<ThreadSource>,
         dynamic_tools: Vec<codex_protocol::dynamic_tools::DynamicToolSpec>,
@@ -1182,6 +1196,7 @@ impl ThreadManagerState {
             auth_manager,
             agent_control,
             self.session_source.clone(),
+            parent_thread_id,
             forked_from_thread_id,
             thread_source,
             dynamic_tools,
@@ -1204,6 +1219,7 @@ impl ThreadManagerState {
         auth_manager: Arc<AuthManager>,
         agent_control: AgentControl,
         session_source: SessionSource,
+        parent_thread_id: Option<ThreadId>,
         forked_from_thread_id: Option<ThreadId>,
         thread_source: Option<ThreadSource>,
         dynamic_tools: Vec<codex_protocol::dynamic_tools::DynamicToolSpec>,
@@ -1258,6 +1274,7 @@ impl ThreadManagerState {
             conversation_history: initial_history,
             session_source,
             forked_from_thread_id,
+            parent_thread_id,
             thread_source,
             agent_control,
             dynamic_tools,