community[patch]: Use get_fields adapter for pydantic (#25191)

Change all usages of __fields__ with get_fields adapter merged into
langchain_core.

Code mod generated using the following grit pattern:

```
engine marzano(0.1)
language python


`$X.__fields__` => `get_fields($X)` where {
    add_import(source="langchain_core.utils.pydantic", name="get_fields")
}
```

libs/community/langchain_community/embeddings/gigachat.py

@@ -7,6 +7,7 @@ from typing import Any, Dict, List, Optional
 from langchain_core.embeddings import Embeddings
 from langchain_core.pydantic_v1 import BaseModel
 from langchain_core.utils import pre_init
+from langchain_core.utils.pydantic import get_fields
 
 logger = logging.getLogger(__name__)
 
@@ -88,7 +89,7 @@ class GigaChatEmbeddings(BaseModel, Embeddings):
                 "Could not import gigachat python package. "
                 "Please install it with `pip install gigachat`."
             )
-        fields = set(cls.__fields__.keys())
+        fields = set(get_fields(cls).keys())
         diff = set(values.keys()) - fields
         if diff:
             logger.warning(f"Extra fields {diff} in GigaChat class")