Make extension lifecycle hooks async (#23291)

## Why

Extension lifecycle hooks sit on the host/extension boundary, but the
current trait surface only allows synchronous callbacks. That forces
extensions that need to seed, rehydrate, observe, or flush
extension-owned state during thread and turn transitions to either block
inside the callback or move async work into separate host plumbing.

This PR makes those lifecycle callbacks awaitable so extension
implementations can perform async work directly at the lifecycle point
where the host already has the relevant session, thread, or turn stores
available.

## What changed

- Makes `ThreadLifecycleContributor` and `TurnLifecycleContributor`
async in `codex-extension-api`.
- Awaits thread start/resume/stop and turn start/stop/abort lifecycle
callbacks from `codex-core`.
- Updates the guardian and memories extensions to implement the async
lifecycle trait surface.
- Updates the existing lifecycle tests to use async contributor
implementations.
- Adds `async-trait` to the crates that now expose or implement these
async object-safe lifecycle traits.

## Testing

- Existing `codex-core` lifecycle tests were updated to cover async
implementations for thread stop and turn abort ordering.

codex-rs/ext/extension-api/src/contributors.rs

@@ -36,16 +36,17 @@ pub trait ContextContributor: Send + Sync {
 /// Implementations should use these callbacks to seed, rehydrate, or flush
 /// extension-private thread state. Heavy dependencies belong on the extension
 /// value created by the host, not in these inputs.
-pub trait ThreadLifecycleContributor<C>: Send + Sync {
+#[async_trait::async_trait]
+pub trait ThreadLifecycleContributor<C: Sync>: Send + Sync {
     /// Called after thread-scoped extension stores are created, before later
     /// contributors can read from them.
-    fn on_thread_start(&self, _input: ThreadStartInput<'_, C>) {}
+    async fn on_thread_start(&self, _input: ThreadStartInput<'_, C>) {}
 
     /// Called after the host constructs a runtime from persisted history.
-    fn on_thread_resume(&self, _input: ThreadResumeInput<'_>) {}
+    async fn on_thread_resume(&self, _input: ThreadResumeInput<'_>) {}
 
     /// Called before the host drops the thread runtime and thread-scoped store.
-    fn on_thread_stop(&self, _input: ThreadStopInput<'_>) {}
+    async fn on_thread_stop(&self, _input: ThreadStopInput<'_>) {}
 }
 
 /// Contributor for host-owned turn lifecycle gates.
@@ -53,16 +54,17 @@ pub trait ThreadLifecycleContributor<C>: Send + Sync {
 /// Implementations should use these callbacks to seed, observe, or clear
 /// extension-private turn state. The host exposes stable identifiers and
 /// extension stores instead of core runtime objects.
+#[async_trait::async_trait]
 pub trait TurnLifecycleContributor: Send + Sync {
     /// Called after turn-scoped extension stores are created, before the task
     /// for the turn starts running.
-    fn on_turn_start(&self, _input: TurnStartInput<'_>) {}
+    async fn on_turn_start(&self, _input: TurnStartInput<'_>) {}
 
     /// Called before the host drops the completed turn runtime and turn store.
-    fn on_turn_stop(&self, _input: TurnStopInput<'_>) {}
+    async fn on_turn_stop(&self, _input: TurnStopInput<'_>) {}
 
     /// Called after the host aborts a running turn.
-    fn on_turn_abort(&self, _input: TurnAbortInput<'_>) {}
+    async fn on_turn_abort(&self, _input: TurnAbortInput<'_>) {}
 }
 
 /// Contributor for host-owned configuration changes.