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/tests/responses_headers.rs

@@ -105,6 +105,7 @@ async fn responses_stream_includes_subagent_header_on_review() {
         /*installation_id*/ TEST_INSTALLATION_ID.to_string(),
         provider.clone(),
         session_source,
+        /*parent_thread_id*/ None,
         config.model_verbosity,
         /*enable_request_compression*/ false,
         /*include_timing_metrics*/ false,
@@ -233,6 +234,7 @@ async fn responses_stream_includes_subagent_header_on_other() {
         /*installation_id*/ TEST_INSTALLATION_ID.to_string(),
         provider.clone(),
         session_source,
+        /*parent_thread_id*/ None,
         config.model_verbosity,
         /*enable_request_compression*/ false,
         /*include_timing_metrics*/ false,
@@ -350,6 +352,7 @@ async fn responses_respects_model_info_overrides_from_config() {
         /*installation_id*/ TEST_INSTALLATION_ID.to_string(),
         provider.clone(),
         session_source,
+        /*parent_thread_id*/ None,
         config.model_verbosity,
         /*enable_request_compression*/ false,
         /*include_timing_metrics*/ false,

codex-rs/core/tests/suite/client.rs

@@ -488,6 +488,7 @@ async fn resume_replays_legacy_js_repl_image_rollout_shapes() {
             item: RolloutItem::SessionMeta(SessionMetaLine {
                 meta: SessionMeta {
                     id: ThreadId::default(),
+                    parent_thread_id: None,
                     timestamp: "2024-01-01T00:00:00Z".to_string(),
                     cwd: ".".into(),
                     originator: "test_originator".to_string(),
@@ -618,6 +619,7 @@ async fn resume_replays_image_tool_outputs_with_detail() {
             item: RolloutItem::SessionMeta(SessionMetaLine {
                 meta: SessionMeta {
                     id: ThreadId::default(),
+                    parent_thread_id: None,
                     timestamp: "2024-01-01T00:00:00Z".to_string(),
                     cwd: ".".into(),
                     originator: "test_originator".to_string(),
@@ -902,6 +904,7 @@ async fn send_provider_auth_request(server: &MockServer, auth: ModelProviderAuth
         /*installation_id*/ "11111111-1111-4111-8111-111111111111".to_string(),
         provider,
         SessionSource::Exec,
+        /*parent_thread_id*/ None,
         config.model_verbosity,
         /*enable_request_compression*/ false,
         /*include_timing_metrics*/ false,
@@ -2358,6 +2361,7 @@ async fn azure_responses_request_includes_store_and_reasoning_ids() {
         /*installation_id*/ "11111111-1111-4111-8111-111111111111".to_string(),
         provider.clone(),
         SessionSource::Exec,
+        /*parent_thread_id*/ None,
         config.model_verbosity,
         /*enable_request_compression*/ false,
         /*include_timing_metrics*/ false,

codex-rs/core/tests/suite/client_websockets.rs

@@ -2155,6 +2155,7 @@ async fn websocket_harness_with_provider_options(
         /*installation_id*/ TEST_INSTALLATION_ID.to_string(),
         provider.clone(),
         SessionSource::Exec,
+        /*parent_thread_id*/ None,
         config.model_verbosity,
         /*enable_request_compression*/ false,
         runtime_metrics_enabled,

codex-rs/core/tests/suite/personality_migration.rs

@@ -61,6 +61,7 @@ async fn write_rollout_with_user_event(dir: &Path, thread_id: ThreadId) -> io::R
         meta: SessionMeta {
             id: thread_id,
             forked_from_id: None,
+            parent_thread_id: None,
             timestamp: TEST_TIMESTAMP.to_string(),
             cwd: std::path::PathBuf::from("."),
             originator: "test_originator".to_string(),
@@ -109,6 +110,7 @@ async fn write_rollout_with_meta_only(dir: &Path, thread_id: ThreadId) -> io::Re
         meta: SessionMeta {
             id: thread_id,
             forked_from_id: None,
+            parent_thread_id: None,
             timestamp: TEST_TIMESTAMP.to_string(),
             cwd: std::path::PathBuf::from("."),
             originator: "test_originator".to_string(),

codex-rs/core/tests/suite/responses_api_proxy_headers.rs

@@ -130,8 +130,9 @@ async fn responses_api_parent_and_subagent_requests_include_identity_headers() -
             .header("x-codex-turn-metadata")
             .ok_or_else(|| anyhow!("child request missing x-codex-turn-metadata"))?,
     )?;
+    assert!(child_turn_metadata.get("forked_from_thread_id").is_none());
     assert_eq!(
-        child_turn_metadata["forked_from_thread_id"].as_str(),
+        child_turn_metadata["parent_thread_id"].as_str(),
         Some(parent_thread_id)
     );
 

codex-rs/core/tests/suite/review.rs

@@ -136,8 +136,9 @@ async fn review_op_emits_lifecycle_and_review_output() {
             .expect("review request turn metadata"),
     )
     .expect("review request turn metadata json");
+    assert!(turn_metadata.get("forked_from_thread_id").is_none());
     assert_eq!(
-        turn_metadata["forked_from_thread_id"].as_str(),
+        turn_metadata["parent_thread_id"].as_str(),
         Some(parent_thread_id.as_str())
     );
 

codex-rs/core/tests/suite/rollout_list_find.rs

@@ -184,6 +184,7 @@ async fn find_locates_rollout_file_written_by_recorder() -> std::io::Result<()>
         RolloutRecorderParams::new(
             thread_id,
             /*forked_from_id*/ None,
+            /*parent_thread_id*/ None,
             SessionSource::Exec,
             /*thread_source*/ None,
             BaseInstructions::default(),

codex-rs/core/tests/suite/sqlite_state.rs

@@ -212,6 +212,7 @@ async fn backfill_scans_existing_rollouts() -> Result<()> {
                 meta: SessionMeta {
                     id: thread_id,
                     forked_from_id: None,
+                    parent_thread_id: None,
                     timestamp: "2026-01-27T12:00:00Z".to_string(),
                     cwd: codex_home.to_path_buf(),
                     originator: "test".to_string(),