Add subagent lineage metadata for responsesapi (#24161)

## Why

We recently added `forked_from_thread_id` which lets us trace where a
thread's _context_ comes from, but we also want to understand subagent
lineage (e.g. which parent thread spawned this subagent? what kind of
subagent is it?) which is orthogonal.

This PR adds `parent_thread_id` and `subagent_kind` to the
`x-codex-turn-metadata` header sent to ResponsesAPI.

## What changed

- Adds `parent_thread_id` and `subagent_kind` to core-owned
`x-codex-turn-metadata`.
- Restores persisted `SessionSource` and `ThreadSource` from resumed
session metadata so cold-resumed subagent threads keep their lineage on
later Responses API requests.
- Centralizes parent-thread extraction on `SessionSource` /
`SubAgentSource` and reuses it in the Responses client, analytics, agent
control, and state parsing paths.
- Extends reserved-key, git-enrichment, thread-spawn, and app-server v2
metadata coverage for the new lineage fields.

## Verification

- Not run locally per request.
- Added focused coverage in `core/src/turn_metadata_tests.rs` and
`app-server/tests/suite/v2/client_metadata.rs`.

codex-rs/analytics/src/events.rs

@@ -1069,12 +1069,9 @@ pub(crate) fn subagent_source_name(subagent_source: &SubAgentSource) -> String {
 }
 
 pub(crate) fn subagent_parent_thread_id(subagent_source: &SubAgentSource) -> Option<String> {
-    match subagent_source {
-        SubAgentSource::ThreadSpawn {
-            parent_thread_id, ..
-        } => Some(parent_thread_id.to_string()),
-        _ => None,
-    }
+    subagent_source
+        .parent_thread_id()
+        .map(|parent_thread_id| parent_thread_id.to_string())
 }
 
 fn analytics_hook_status(status: HookRunStatus) -> HookRunStatus {