Revert "Gemini client support for messages without role (A2A) (#2358)"

This reverts commit de13b2f.

Author: priyansh4320

autogen/oai/gemini.py

@@ -632,7 +632,7 @@ def _oai_messages_to_gemini_messages(self, messages: list[dict[str, Any]]) -> li
         rst = []
         for message in messages:
             parts, part_type = self._oai_content_to_gemini_content(message)
-            role = "user" if message.get("role", "user") in ["user", "system"] else "model"
+            role = "user" if message["role"] in ["user", "system"] else "model"
 
             if part_type == "text":
                 rst.append(

test/oai/test_gemini.py

@@ -223,23 +223,6 @@ def test_gemini_empty_message_handling(self, gemini_client):
         assert converted_messages[-3].parts[0].text == "empty", "Empty message is not converted to 'empty' correctly"
         assert converted_messages[-1].parts[0].text == "empty", "Empty message is not converted to 'empty' correctly"
 
-    def test_gemini_message_without_role_defaults_to_user(self, gemini_client):
-        """Test that messages without a 'role' field default to 'user' role (e.g., A2A messages)."""
-        messages = [
-            {"content": "Hello, this message has no role field"},
-            {"role": "model", "content": "How can I help you?"},
-            {"content": "Another message without role"},
-        ]
-
-        converted_messages = gemini_client._oai_messages_to_gemini_messages(messages)
-
-        # Messages without role should be treated as "user"
-        assert converted_messages[0].role == "user", "Message without role should default to 'user'"
-        assert converted_messages[0].parts[0].text == "Hello, this message has no role field"
-        assert converted_messages[1].role == "model"
-        assert converted_messages[2].role == "user", "Message without role should default to 'user'"
-        assert converted_messages[2].parts[0].text == "Another message without role"
-
     def test_vertexai_safety_setting_conversion(self):
         safety_settings = [
             {"category": "HARM_CATEGORY_HARASSMENT", "threshold": "BLOCK_ONLY_HIGH"},