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/features.rs

@@ -137,6 +137,8 @@ pub enum Feature {
     SkillApproval,
     /// Steer feature flag - when enabled, Enter submits immediately instead of queuing.
     Steer,
+    /// Allow request_user_input in Default collaboration mode.
+    DefaultModeRequestUserInput,
     /// Enable collaboration modes (Plan, Default).
     /// Kept for config backward compatibility; behavior is always collaboration-modes-enabled.
     CollaborationModes,
@@ -639,6 +641,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::Stable,
         default_enabled: true,
     },
+    FeatureSpec {
+        id: Feature::DefaultModeRequestUserInput,
+        key: "default_mode_request_user_input",
+        stage: Stage::UnderDevelopment,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::CollaborationModes,
         key: "collaboration_modes",