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/turn_context.rs

@@ -5,6 +5,7 @@ use crate::environment_selection::ResolvedTurnEnvironments;
 use codex_model_provider::SharedModelProvider;
 use codex_model_provider::create_model_provider;
 use codex_protocol::SessionId;
+use codex_protocol::ThreadId;
 use codex_protocol::models::AdditionalPermissionProfile;
 use codex_protocol::openai_models::ToolMode;
 use codex_protocol::protocol::ThreadSource;
@@ -62,6 +63,7 @@ pub struct TurnContext {
     pub(crate) reasoning_effort: Option<ReasoningEffortConfig>,
     pub(crate) reasoning_summary: ReasoningSummaryConfig,
     pub(crate) session_source: SessionSource,
+    pub(crate) parent_thread_id: Option<ThreadId>,
     pub(crate) thread_source: Option<ThreadSource>,
     pub(crate) environments: ResolvedTurnEnvironments,
     /// The session's absolute working directory. All relative paths provided
@@ -232,6 +234,7 @@ impl TurnContext {
             reasoning_effort,
             reasoning_summary: self.reasoning_summary,
             session_source: self.session_source.clone(),
+            parent_thread_id: self.parent_thread_id,
             thread_source: self.thread_source,
             environments: self.environments.clone(),
             #[allow(deprecated)]
@@ -506,6 +509,7 @@ impl Session {
             session_id.to_string(),
             thread_id.to_string(),
             session_configuration.forked_from_thread_id,
+            session_configuration.parent_thread_id,
             &session_configuration.session_source,
             session_configuration.thread_source,
             sub_id.clone(),
@@ -529,6 +533,7 @@ impl Session {
             reasoning_effort,
             reasoning_summary,
             session_source,
+            parent_thread_id: session_configuration.parent_thread_id,
             thread_source: session_configuration.thread_source,
             environments,
             #[allow(deprecated)]