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/message_processor.rs

@@ -49,6 +49,7 @@ use codex_core::default_client::USER_AGENT_SUFFIX;
 use codex_core::default_client::get_codex_user_agent;
 use codex_core::default_client::set_default_client_residency_requirement;
 use codex_core::default_client::set_default_originator;
+use codex_core::models_manager::collaboration_mode_presets::CollaborationModesConfig;
 use codex_feedback::CodexFeedback;
 use codex_protocol::ThreadId;
 use codex_protocol::protocol::SessionSource;
@@ -182,6 +183,11 @@ impl MessageProcessor {
             auth_manager.clone(),
             SessionSource::VSCode,
             config.model_catalog.clone(),
+            CollaborationModesConfig {
+                default_mode_request_user_input: config
+                    .features
+                    .enabled(codex_core::features::Feature::DefaultModeRequestUserInput),
+            },
         ));
         let cloud_requirements = Arc::new(RwLock::new(cloud_requirements));
         let codex_message_processor = CodexMessageProcessor::new(CodexMessageProcessorArgs {