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/test_support.rs

@@ -85,5 +85,7 @@ pub fn all_model_presets() -> &'static Vec<ModelPreset> {
 }
 
 pub fn builtin_collaboration_mode_presets() -> Vec<CollaborationModeMask> {
-    collaboration_mode_presets::builtin_collaboration_mode_presets()
+    collaboration_mode_presets::builtin_collaboration_mode_presets(
+        collaboration_mode_presets::CollaborationModesConfig::default(),
+    )
 }