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/codex.rs

@@ -29,6 +29,8 @@ use crate::features::FEATURES;
 use crate::features::Feature;
 use crate::features::Features;
 use crate::features::maybe_push_unstable_features_warning;
+#[cfg(test)]
+use crate::models_manager::collaboration_mode_presets::CollaborationModesConfig;
 use crate::models_manager::manager::ModelsManager;
 use crate::parse_command::parse_command;
 use crate::parse_turn_item;
@@ -8293,6 +8295,7 @@ mod tests {
             config.codex_home.clone(),
             auth_manager.clone(),
             None,
+            CollaborationModesConfig::default(),
         ));
         let model = ModelsManager::get_model_offline_for_tests(config.model.as_deref());
         let model_info =
@@ -8369,6 +8372,7 @@ mod tests {
             config.codex_home.clone(),
             auth_manager.clone(),
             None,
+            CollaborationModesConfig::default(),
         ));
         let agent_control = AgentControl::default();
         let exec_policy = ExecPolicyManager::default();
@@ -8527,6 +8531,7 @@ mod tests {
             config.codex_home.clone(),
             auth_manager.clone(),
             None,
+            CollaborationModesConfig::default(),
         ));
         let agent_control = AgentControl::default();
         let exec_policy = ExecPolicyManager::default();