Route Guardian reviewers through ThreadManager for inline parents (#45518)

## Why

Inline delegates have no entry in the thread registry, so reviewer creation cannot depend on looking up the parent or waiting for its thread-ready notification.

## What changed

- Capture parent identity, authentication, shared agent control, originator, and inherited instructions in `StartThreadOptions` so `ThreadManager` can start a child without a registered parent.
- Route Guardian reviewer creation through this path, remove the standalone fallback and readiness gate, and require a Guardian extension host.
- Install explicit reviewer hosts in unit tests and the Guardian reviewer extension in the integration test harness, using `ExtensionRegistry::to_builder()` to preserve existing contributors.

## Testing

Extend the thread-manager regression test to remove the parent from the registry before starting a child, then verify inherited lineage, originator, session identity, and authentication, and exclusion from the public thread list.

GitOrigin-RevId: 468ded6fdce7520cb39d55c1a884dcfa5aaea2d9

codex-rs/core/src/codex_delegate.rs

@@ -81,11 +81,8 @@ pub(crate) async fn run_codex_thread_interactive(
     } else {
         Arc::clone(&parent_session.services.extensions)
     };
-    // Inline delegates never register with ThreadManager or receive on_thread_ready.
-    // Bind their standalone spawn path before inherited extensions run.
     let mut thread_extension_init = codex_extension_api::ExtensionDataInit::default();
     thread_extension_init.insert(isolation);
-    thread_extension_init.insert(crate::guardian::GuardianReviewSessionHost::default());
     let (session, io) = Session::spawn(SessionSpawnArgs {
         startup: None,
         config,