[Fix] Add disable_custom_api_keys to UISettings Pydantic model

Without this field on the model, GET /get/ui_settings omits the setting
from the response and field_schema, preventing the UI from reading it.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

litellm/proxy/ui_crud_endpoints/proxy_setting_endpoints.py

@@ -129,6 +129,11 @@ class UISettings(BaseModel):
         description="If enabled, the user search endpoint (/user/filter/ui) restricts results by organization. When off, any authenticated user can search all users.",
     )
 
+    disable_custom_api_keys: bool = Field(
+        default=False,
+        description="If true, users cannot specify custom API key values. All keys must be auto-generated.",
+    )
+
 
 class UISettingsResponse(SettingsResponse):
     """Response model for UI settings"""