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/mcp-server/src/message_processor.rs

@@ -6,6 +6,7 @@ use codex_core::ThreadManager;
 use codex_core::config::Config;
 use codex_core::default_client::USER_AGENT_SUFFIX;
 use codex_core::default_client::get_codex_user_agent;
+use codex_core::models_manager::collaboration_mode_presets::CollaborationModesConfig;
 use codex_protocol::ThreadId;
 use codex_protocol::protocol::SessionSource;
 use codex_protocol::protocol::Submission;
@@ -62,6 +63,11 @@ impl MessageProcessor {
             auth_manager,
             SessionSource::Mcp,
             config.model_catalog.clone(),
+            CollaborationModesConfig {
+                default_mode_request_user_input: config
+                    .features
+                    .enabled(codex_core::features::Feature::DefaultModeRequestUserInput),
+            },
         ));
         Self {
             outgoing,