Reject ask user question tool in Execute and Custom (#9560)

## Summary
- Keep `request_user_input` in the tool list but reject it at runtime in
Execute/Custom modes with a clear model-facing error.
- Add a session accessor for current collaboration mode and enforce the
gate in the request_user_input handler.
- Update core/app-server tests to use Plan mode for success and add
Execute/Custom rejection coverage.

codex-rs/core/src/codex.rs

@@ -1531,6 +1531,11 @@ impl Session {
         self.features.clone()
     }
 
+    pub(crate) async fn collaboration_mode(&self) -> CollaborationMode {
+        let state = self.state.lock().await;
+        state.session_configuration.collaboration_mode.clone()
+    }
+
     async fn send_raw_response_items(&self, turn_context: &TurnContext, items: &[ResponseItem]) {
         for item in items {
             self.send_event(