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/app-server/tests/common/lib.rs

@@ -38,6 +38,7 @@ pub use responses::create_final_assistant_message_sse_response;
 pub use responses::create_request_permissions_sse_response;
 pub use responses::create_request_user_input_sse_response;
 pub use responses::create_shell_command_sse_response;
+pub use rollout::create_fake_parented_rollout_with_source;
 pub use rollout::create_fake_rollout;
 pub use rollout::create_fake_rollout_with_source;
 pub use rollout::create_fake_rollout_with_text_elements;

codex-rs/app-server/tests/common/rollout.rs

@@ -118,6 +118,53 @@ pub fn create_fake_rollout_with_source(
     model_provider: Option<&str>,
     git_info: Option<GitInfo>,
     source: SessionSource,
+) -> Result<String> {
+    create_fake_rollout_with_source_and_parent_thread_id(
+        codex_home,
+        filename_ts,
+        meta_rfc3339,
+        preview,
+        model_provider,
+        git_info,
+        source,
+        /*parent_thread_id*/ None,
+    )
+}
+
+/// Create a minimal rollout file with an explicit session source and control parent.
+#[allow(clippy::too_many_arguments)]
+pub fn create_fake_parented_rollout_with_source(
+    codex_home: &Path,
+    filename_ts: &str,
+    meta_rfc3339: &str,
+    preview: &str,
+    model_provider: Option<&str>,
+    git_info: Option<GitInfo>,
+    source: SessionSource,
+    parent_thread_id: ThreadId,
+) -> Result<String> {
+    create_fake_rollout_with_source_and_parent_thread_id(
+        codex_home,
+        filename_ts,
+        meta_rfc3339,
+        preview,
+        model_provider,
+        git_info,
+        source,
+        Some(parent_thread_id),
+    )
+}
+
+#[allow(clippy::too_many_arguments)]
+fn create_fake_rollout_with_source_and_parent_thread_id(
+    codex_home: &Path,
+    filename_ts: &str,
+    meta_rfc3339: &str,
+    preview: &str,
+    model_provider: Option<&str>,
+    git_info: Option<GitInfo>,
+    source: SessionSource,
+    parent_thread_id: Option<ThreadId>,
 ) -> Result<String> {
     let uuid = Uuid::new_v4();
     let uuid_str = uuid.to_string();
@@ -133,6 +180,7 @@ pub fn create_fake_rollout_with_source(
     let meta = SessionMeta {
         id: conversation_id,
         forked_from_id: None,
+        parent_thread_id,
         timestamp: meta_rfc3339.to_string(),
         cwd: PathBuf::from("/"),
         originator: "codex".to_string(),
@@ -217,6 +265,7 @@ pub fn create_fake_rollout_with_text_elements(
     let meta = SessionMeta {
         id: conversation_id,
         forked_from_id: None,
+        parent_thread_id: None,
         timestamp: meta_rfc3339.to_string(),
         cwd: PathBuf::from("/"),
         originator: "codex".to_string(),