Enable request_user_input in Default mode (#12735)

## Summary
- allow `request_user_input` in Default collaboration mode as well as
Plan
- update the Default-mode instructions to prefer assumptions first and
use `request_user_input` only when a question is unavoidable
- update request_user_input and app-server tests to match the new
Default-mode behavior
- refactor collaboration-mode availability plumbing into
`CollaborationModesConfig` for future mode-related flags

## Codex author
`codex resume 019c9124-ed28-7c13-96c6-b916b1c97d49`

codex-rs/app-server/src/codex_message_processor.rs

@@ -222,6 +222,7 @@ use codex_core::find_thread_path_by_id_str;
 use codex_core::git_info::git_diff_to_remote;
 use codex_core::mcp::collect_mcp_snapshot;
 use codex_core::mcp::group_tools_by_server;
+use codex_core::models_manager::collaboration_mode_presets::CollaborationModesConfig;
 use codex_core::parse_cursor;
 use codex_core::read_head_for_summary;
 use codex_core::read_session_meta_line;
@@ -479,11 +480,13 @@ impl CodexMessageProcessor {
     fn normalize_turn_start_collaboration_mode(
         &self,
         mut collaboration_mode: CollaborationMode,
+        collaboration_modes_config: CollaborationModesConfig,
     ) -> CollaborationMode {
         if collaboration_mode.settings.developer_instructions.is_none()
             && let Some(instructions) = self
                 .thread_manager
-                .list_collaboration_modes()
+                .get_models_manager()
+                .list_collaboration_modes_for_config(collaboration_modes_config)
                 .into_iter()
                 .find(|preset| preset.mode == Some(collaboration_mode.mode))
                 .and_then(|preset| preset.developer_instructions.flatten())
@@ -3909,7 +3912,11 @@ impl CodexMessageProcessor {
         params: CollaborationModeListParams,
     ) {
         let CollaborationModeListParams {} = params;
-        let items = thread_manager.list_collaboration_modes();
+        let items = thread_manager
+            .list_collaboration_modes()
+            .into_iter()
+            .map(Into::into)
+            .collect();
         let response = CollaborationModeListResponse { data: items };
         outgoing.send_response(request_id, response).await;
     }
@@ -5568,9 +5575,12 @@ impl CodexMessageProcessor {
             }
         };
 
-        let collaboration_mode = params
-            .collaboration_mode
-            .map(|mode| self.normalize_turn_start_collaboration_mode(mode));
+        let collaboration_modes_config = CollaborationModesConfig {
+            default_mode_request_user_input: thread.enabled(Feature::DefaultModeRequestUserInput),
+        };
+        let collaboration_mode = params.collaboration_mode.map(|mode| {
+            self.normalize_turn_start_collaboration_mode(mode, collaboration_modes_config)
+        });
 
         // Map v2 input items to core input items.
         let mapped_items: Vec<CoreInputItem> = params