Fix pydantic model_fields deprecation warning

Access model_fields from class instead of instance:
- self.model_fields -> type(self).model_fields

Reduces test warnings from 3 to 1 (remaining is intentional user warning).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: dimitri-yatsenko

src/datajoint/settings.py

@@ -827,10 +827,10 @@ def __setitem__(self, key: str, value: Any) -> None:
 
     def __delitem__(self, key: str) -> None:
         """Reset setting to default by dot-notation key."""
-        # Get the default value from the model fields
+        # Get the default value from the model fields (access from class, not instance)
         parts = key.split(".")
         if len(parts) == 1:
-            field_info = self.model_fields.get(key)
+            field_info = type(self).model_fields.get(key)
             if field_info is not None:
                 default = field_info.default
                 if default is not None: