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/retrievers/docarray.py

@@ -6,6 +6,7 @@ from langchain_core.callbacks import CallbackManagerForRetrieverRun
 from langchain_core.documents import Document
 from langchain_core.embeddings import Embeddings
 from langchain_core.retrievers import BaseRetriever
+from langchain_core.utils.pydantic import get_fields
 
 from langchain_community.vectorstores.utils import maximal_marginal_relevance
 
@@ -182,7 +183,7 @@ class DocArrayRetriever(BaseRetriever):
             ValueError: If the document doesn't contain the content field
         """
 
-        fields = doc.keys() if isinstance(doc, dict) else doc.__fields__
+        fields = doc.keys() if isinstance(doc, dict) else get_fields(doc)
 
         if self.content_field not in fields:
             raise ValueError(