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-protocol/schema/json/codex_app_server_protocol.schemas.json

@@ -8150,14 +8150,8 @@
         "type": "object"
       },
       "CollaborationModeMask": {
-        "description": "A mask for collaboration mode settings, allowing partial updates. All fields except `name` are optional, enabling selective updates.",
+        "description": "EXPERIMENTAL - collaboration mode preset metadata for clients.",
         "properties": {
-          "developer_instructions": {
-            "type": [
-              "string",
-              "null"
-            ]
-          },
           "mode": {
             "anyOf": [
               {