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/app-server/tests/suite/v2/request_user_input.rs

@@ -12,6 +12,8 @@ use codex_app_server_protocol::ThreadStartResponse;
 use codex_app_server_protocol::TurnStartParams;
 use codex_app_server_protocol::TurnStartResponse;
 use codex_app_server_protocol::UserInput as V2UserInput;
+use codex_protocol::config_types::CollaborationMode;
+use codex_protocol::config_types::Settings;
 use codex_protocol::openai_models::ReasoningEffort;
 use tokio::time::timeout;
 
@@ -52,6 +54,11 @@ async fn request_user_input_round_trip() -> Result<()> {
             }],
             model: Some("mock-model".to_string()),
             effort: Some(ReasoningEffort::Medium),
+            collaboration_mode: Some(CollaborationMode::Plan(Settings {
+                model: "mock-model".to_string(),
+                reasoning_effort: Some(ReasoningEffort::Medium),
+                developer_instructions: None,
+            })),
             ..Default::default()
         })
         .await?;