fix(guardian, app-server): introduce guardian review ids (#17298)

## Description

This PR introduces `review_id` as the stable identifier for guardian
reviews and exposes it in app-server `item/autoApprovalReview/started`
and `item/autoApprovalReview/completed` events.

Internally, guardian rejection state is now keyed by `review_id` instead
of the reviewed tool item ID. `target_item_id` is still included when a
review maps to a concrete thread item, but it is no longer overloaded as
the review lifecycle identifier.

## Motivation

We'd like to give users the ability to preempt a guardian review while
it's running (approve or decline).

However, we can't implement the API that allows the user to override a
running guardian review because we didn't have a unique `review_id` per
guardian review. Using `target_item_id` is not correct since:
- with execve reviews, there can be multiple execve calls (and therefore
guardian reviews) per shell command
- with network policy reviews, there is no target item ID

The PR that actually implements user overrides will use `review_id` as
the stable identifier.

codex-rs/core/src/codex.rs

@@ -2026,7 +2026,7 @@ impl Session {
             session_telemetry,
             models_manager: Arc::clone(&models_manager),
             tool_approvals: Mutex::new(ApprovalStore::default()),
-            guardian_rejection_rationales: Mutex::new(HashMap::new()),
+            guardian_rejections: Mutex::new(HashMap::new()),
             skills_manager,
             plugins_manager: Arc::clone(&plugins_manager),
             mcp_manager: Arc::clone(&mcp_manager),