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/core/src/thread_manager.rs

@@ -11,6 +11,7 @@ use crate::error::CodexErr;
 use crate::error::Result as CodexResult;
 use crate::file_watcher::FileWatcher;
 use crate::file_watcher::FileWatcherEvent;
+use crate::models_manager::collaboration_mode_presets::CollaborationModesConfig;
 use crate::models_manager::manager::ModelsManager;
 use crate::protocol::Event;
 use crate::protocol::EventMsg;
@@ -143,6 +144,7 @@ impl ThreadManager {
         auth_manager: Arc<AuthManager>,
         session_source: SessionSource,
         model_catalog: Option<ModelsResponse>,
+        collaboration_modes_config: CollaborationModesConfig,
     ) -> Self {
         let (thread_created_tx, _) = broadcast::channel(THREAD_CREATED_CHANNEL_CAPACITY);
         let skills_manager = Arc::new(SkillsManager::new(codex_home.clone()));
@@ -155,6 +157,7 @@ impl ThreadManager {
                     codex_home,
                     auth_manager.clone(),
                     model_catalog,
+                    collaboration_modes_config,
                 )),
                 skills_manager,
                 file_watcher,