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/tests/integration_tests/llms/utils.py

@@ -1,6 +1,7 @@
 """Utils for LLM Tests."""
 
 from langchain_core.language_models.llms import BaseLLM
+from langchain_core.utils.pydantic import get_fields
 
 
 def assert_llm_equality(llm: BaseLLM, loaded_llm: BaseLLM) -> None:
@@ -9,7 +10,7 @@ def assert_llm_equality(llm: BaseLLM, loaded_llm: BaseLLM) -> None:
     assert type(llm) is type(loaded_llm)
     # Client field can be session based, so hash is different despite
     # all other values being the same, so just assess all other fields
-    for field in llm.__fields__.keys():
+    for field in get_fields(llm).keys():
         if field != "client" and field != "pipeline":
             val = getattr(llm, field)
             new_val = getattr(loaded_llm, field)