feat(core) Introduce Feature::RequestPermissions (#11871)

## Summary
Introduces the initial implementation of Feature::RequestPermissions.
RequestPermissions allows the model to request that a command be run
inside the sandbox, with additional permissions, like writing to a
specific folder. Eventually this will include other rules as well, and
the ability to persist these permissions, but this PR is already quite
large - let's get the core flow working and go from there!

<img width="1279" height="541" alt="Screenshot 2026-02-15 at 2 26 22 PM"
src="https://github.com/user-attachments/assets/0ee3ec0f-02ec-4509-91a2-809ac80be368"
/>

## Testing
- [x] Added tests
- [x] Tested locally
- [x] Feature

codex-rs/core/src/codex_delegate.rs

@@ -320,6 +320,7 @@ async fn handle_exec_approval(
         reason,
         network_approval_context,
         proposed_execpolicy_amendment,
+        additional_permissions,
         ..
     } = event;
     // Race approval with cancellation and timeout to avoid hangs.
@@ -332,6 +333,7 @@ async fn handle_exec_approval(
         reason,
         network_approval_context,
         proposed_execpolicy_amendment,
+        additional_permissions,
     );
     let decision = await_approval_with_cancel(
         approval_fut,