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/tools/handlers/agent_jobs.rs

@@ -211,6 +211,7 @@ async fn run_agent_job_loop(
                             "agent_job:{job_id}"
                         )))),
                         SpawnAgentOptions {
+                            parent_thread_id: Some(session.conversation_id),
                             environments: Some(turn.environments.to_selections()),
                             ..Default::default()
                         },

codex-rs/core/src/tools/handlers/multi_agents/spawn.rs

@@ -124,6 +124,7 @@ async fn handle_spawn_agent(
         SpawnAgentOptions {
             fork_parent_spawn_call_id: args.fork_context.then(|| call_id.clone()),
             fork_mode: args.fork_context.then_some(SpawnAgentForkMode::FullHistory),
+            parent_thread_id: Some(session.conversation_id),
             environments: Some(turn.environments.to_selections()),
         },
     ))

codex-rs/core/src/tools/handlers/multi_agents_v2/list_agents.rs

@@ -30,7 +30,7 @@ impl ToolExecutor<ToolInvocation> for Handler {
         session
             .services
             .agent_control
-            .register_session_root(session.conversation_id, &turn.session_source);
+            .register_session_root(session.conversation_id, turn.parent_thread_id);
         let agents = session
             .services
             .agent_control

codex-rs/core/src/tools/handlers/multi_agents_v2/spawn.rs

@@ -144,6 +144,7 @@ async fn handle_spawn_agent(
             SpawnAgentOptions {
                 fork_parent_spawn_call_id: fork_mode.as_ref().map(|_| call_id.clone()),
                 fork_mode,
+                parent_thread_id: Some(session.conversation_id),
                 environments: Some(turn.environments.to_selections()),
             },
         ),