Set default region from boto3 session for Bedrock (langchain-ai#13694)

- **Description:** Set default region from boto3 session for Bedrock 
- **Issue:** langchain-ai#13683

libs/langchain/langchain/llms/bedrock.py

@@ -204,7 +204,7 @@ def validate_environment(cls, values: Dict) -> Dict:
                 values,
                 "region_name",
                 "AWS_DEFAULT_REGION",
-                default=None,
+                default=session.region_name,
             )
 
             client_params = {}