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/config.schema.json

@@ -328,6 +328,9 @@
             "connectors": {
               "type": "boolean"
             },
+            "default_mode_request_user_input": {
+              "type": "boolean"
+            },
             "elevated_windows_sandbox": {
               "type": "boolean"
             },
@@ -1621,6 +1624,9 @@
         "connectors": {
           "type": "boolean"
         },
+        "default_mode_request_user_input": {
+          "type": "boolean"
+        },
         "elevated_windows_sandbox": {
           "type": "boolean"
         },