chore: access model_fields from class instead of instance (#800)

## Summary

- Access `model_fields` from the class
(`type(configuration).model_fields`) instead of the instance
(`configuration.model_fields`) in `Configuration.from_configuration`

Since Pydantic V2.11, accessing `model_fields` on an instance emits a
`PydanticDeprecatedSince211` deprecation warning (to be removed in
V3.0).

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: vdusek

src/apify/_configuration.py

@@ -505,7 +505,7 @@ def from_configuration(cls, configuration: CrawleeConfiguration) -> Configuratio
         # Due to known Pydantic issue https://github.com/pydantic/pydantic/issues/9516, creating new instance of
         # Configuration from existing one in situation where environment can have some fields set by alias is very
         # unpredictable. Use the stable workaround.
-        for name in configuration.model_fields:
+        for name in type(configuration).model_fields:
             setattr(apify_configuration, name, getattr(configuration, name))
 
         return apify_configuration