Fix multiturn conversation error Groq

* Groq assistant turns can only have `content: <str>`, no arrays or multimodal
* Groq cannot handle response_format argument the way that its passed, I'm working on a separate commit for that.

src/ell/providers/groq.py

@@ -13,6 +13,8 @@ class GroqProvider(OpenAIProvider):
         def translate_to_provider(self, *args, **kwargs):
             params = super().translate_to_provider(*args, **kwargs)
             params.pop('stream_options', None)
+            assert 'response_format' not in params, 'Groq does not support response_format.'
+            params['messages'] = messages_to_groq_message_format(params['messages'])
             return params
 
         def translate_from_provider(self, *args, **kwargs):
@@ -24,3 +26,22 @@ def translate_from_provider(self, *args, **kwargs):
 except ImportError:
     pass
 
+def messages_to_groq_message_format(messages):
+    """Assistant messages to Groq must take the format: {'role': 'assistant', 'content': <string>}"""
+    # XXX: Issue #289: groq.BadRequestError: Error code: 400 - {'error': {'message': "'messages.1' : for 'role:assistant' the following must be satisfied[('messages.1.content' : value must be a string)]", 'type': 'invalid_request_error'}}
+    new_messages = []
+    for message in messages:
+        if message['role'] == 'assistant':
+            # Assistant messages must be strings
+            # If content is a list, only one string element is allowed
+            if isinstance(message['content'], str):
+                new_messages.append({'role': 'assistant', 'content': message['content']})
+            elif isinstance(message['content'], list) and len(message['content']) == 1 and message['content'][0]['type'] == 'text':
+                new_messages.append({'role': 'assistant', 'content': message['content'][0]['text']})
+            else:
+                raise ValueError('Groq assistant messages must contain exactly one string content.')
+        else:
+            new_messages.append(message)
+
+    return new_messages
+