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/mcp-server/src/codex_tool_runner.rs

@@ -226,6 +226,7 @@ async fn run_codex_tool_session_inner(
                             proposed_network_policy_amendments: _,
                             parsed_cmd,
                             network_approval_context: _,
+                            additional_permissions: _,
                         } = ev;
                         handle_exec_approval_request(
                             command,