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/tests/suite/v2/request_user_input.rs

@@ -123,9 +123,6 @@ sandbox_mode = "read-only"
 
 model_provider = "mock_provider"
 
-[features]
-collaboration_modes = true
-
 [model_providers.mock_provider]
 name = "Mock provider for test"
 base_url = "{server_uri}/v1"