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/rollout/src/tests.rs

@@ -606,6 +606,7 @@ async fn test_list_conversations_latest_first() {
                 git_sha: None,
                 git_origin_url: None,
                 source: Some(SessionSource::VSCode),
+                parent_thread_id: None,
                 agent_nickname: None,
                 agent_role: None,
                 model_provider: Some(TEST_PROVIDER.to_string()),
@@ -623,6 +624,7 @@ async fn test_list_conversations_latest_first() {
                 git_sha: None,
                 git_origin_url: None,
                 source: Some(SessionSource::VSCode),
+                parent_thread_id: None,
                 agent_nickname: None,
                 agent_role: None,
                 model_provider: Some(TEST_PROVIDER.to_string()),
@@ -640,6 +642,7 @@ async fn test_list_conversations_latest_first() {
                 git_sha: None,
                 git_origin_url: None,
                 source: Some(SessionSource::VSCode),
+                parent_thread_id: None,
                 agent_nickname: None,
                 agent_role: None,
                 model_provider: Some(TEST_PROVIDER.to_string()),
@@ -750,6 +753,7 @@ async fn test_pagination_cursor() {
                 git_sha: None,
                 git_origin_url: None,
                 source: Some(SessionSource::VSCode),
+                parent_thread_id: None,
                 agent_nickname: None,
                 agent_role: None,
                 model_provider: Some(TEST_PROVIDER.to_string()),
@@ -767,6 +771,7 @@ async fn test_pagination_cursor() {
                 git_sha: None,
                 git_origin_url: None,
                 source: Some(SessionSource::VSCode),
+                parent_thread_id: None,
                 agent_nickname: None,
                 agent_role: None,
                 model_provider: Some(TEST_PROVIDER.to_string()),
@@ -820,6 +825,7 @@ async fn test_pagination_cursor() {
                 git_sha: None,
                 git_origin_url: None,
                 source: Some(SessionSource::VSCode),
+                parent_thread_id: None,
                 agent_nickname: None,
                 agent_role: None,
                 model_provider: Some(TEST_PROVIDER.to_string()),
@@ -837,6 +843,7 @@ async fn test_pagination_cursor() {
                 git_sha: None,
                 git_origin_url: None,
                 source: Some(SessionSource::VSCode),
+                parent_thread_id: None,
                 agent_nickname: None,
                 agent_role: None,
                 model_provider: Some(TEST_PROVIDER.to_string()),
@@ -882,6 +889,7 @@ async fn test_pagination_cursor() {
             git_sha: None,
             git_origin_url: None,
             source: Some(SessionSource::VSCode),
+            parent_thread_id: None,
             agent_nickname: None,
             agent_role: None,
             model_provider: Some(TEST_PROVIDER.to_string()),
@@ -1052,6 +1060,7 @@ async fn test_get_thread_contents() {
             git_sha: None,
             git_origin_url: None,
             source: Some(SessionSource::VSCode),
+            parent_thread_id: None,
             agent_nickname: None,
             agent_role: None,
             model_provider: Some(TEST_PROVIDER.to_string()),
@@ -1251,6 +1260,7 @@ async fn test_updated_at_uses_file_mtime() -> Result<()> {
             meta: SessionMeta {
                 id: conversation_id,
                 forked_from_id: None,
+                parent_thread_id: None,
                 timestamp: ts.to_string(),
                 cwd: ".".into(),
                 originator: "test_originator".into(),
@@ -1403,6 +1413,7 @@ async fn test_timestamp_only_cursor_skips_same_second_filesystem_ties() {
                 git_sha: None,
                 git_origin_url: None,
                 source: Some(SessionSource::VSCode),
+                parent_thread_id: None,
                 agent_nickname: None,
                 agent_role: None,
                 model_provider: Some(TEST_PROVIDER.to_string()),
@@ -1420,6 +1431,7 @@ async fn test_timestamp_only_cursor_skips_same_second_filesystem_ties() {
                 git_sha: None,
                 git_origin_url: None,
                 source: Some(SessionSource::VSCode),
+                parent_thread_id: None,
                 agent_nickname: None,
                 agent_role: None,
                 model_provider: Some(TEST_PROVIDER.to_string()),