Add forked_from_thread_id turn metadata (#24160)

## Why

When Codex calls responsesapi, we currently send `session_id`,
`thread_id`, and `turn_id` among other things as
`client_metadata["x-codex-turn-metadata"]`. This PR adds
`forked_from_thread_id` which helps explain the "lineage" of a forked
thread.

## What's changed

- Track the immediate history source copied into a forked thread through
thread/session creation, including subagent and review turn metadata
paths.
- Include `forked_from_thread_id` in Codex turn metadata while
preventing turn-scoped Responses API client metadata from overwriting
Codex-owned lineage fields.
- Add coverage for fork lineage in turn metadata and the app-server
Responses API request path.

codex-rs/core/src/session/turn_context.rs

@@ -482,6 +482,7 @@ impl Session {
         let turn_metadata_state = Arc::new(TurnMetadataState::new(
             session_id.to_string(),
             thread_id.to_string(),
+            session_configuration.forked_from_thread_id,
             session_configuration.thread_source,
             sub_id.clone(),
             cwd.clone(),