Store turn skill state in extension data (#36740)

## What changed

- Store each turn's host skills snapshot in `ExtensionData` and expose it through
  `TurnContext::skills_snapshot`.
- Lazily keep implicit skill invocation deduplication state in the same turn-scoped
  store.
- Carry the skills snapshot into review turns without a separate
  `TurnSkillsContext` field.

GitOrigin-RevId: d71579b5d230bd0b5f3c2ad062861f74d614d88c

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

@@ -106,7 +106,7 @@ pub(super) async fn spawn_review_thread(
     let extension_data = Arc::new(codex_extension_api::ExtensionData::new(
         review_turn_id.clone(),
     ));
-    extension_data.insert(parent_turn_context.turn_skills.snapshot.clone());
+    extension_data.insert(parent_turn_context.skills_snapshot().as_ref().clone());
 
     let review_turn_context = TurnContext {
         sub_id: review_turn_id.clone(),
@@ -147,7 +147,6 @@ pub(super) async fn spawn_review_thread(
         dynamic_tools: parent_turn_context.dynamic_tools.clone(),
         turn_metadata_state,
         extension_data,
-        turn_skills: TurnSkillsContext::new(parent_turn_context.turn_skills.snapshot.clone()),
         turn_timing_state: Arc::new(TurnTimingState::default()),
         terminal_error: Arc::new(Mutex::new(None)),
         server_model_warning_emitted: AtomicBool::new(false),