Python: response format fix (microsoft#4479)

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### Contribution Checklist

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- [ ] The code builds clean without any errors or warnings
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Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
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raises no violations
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python/semantic_kernel/connectors/ai/open_ai/request_settings/open_ai_request_settings.py

@@ -54,7 +54,7 @@ def check_best_of_and_n(self) -> "OpenAITextRequestSettings":
 class OpenAIChatRequestSettings(OpenAIRequestSettings):
     """Specific settings for the Chat Completion endpoint."""
 
-    response_format: Optional[Literal["text", "json_object"]] = "text"
+    response_format: Optional[Dict[Literal["type"], Literal["text", "json_object"]]] = None
     tools: Optional[List[Dict[str, Any]]] = None
     tool_choice: Optional[str] = None
     function_call: Optional[str] = None

python/tests/unit/ai/open_ai/test_openai_request_settings.py

@@ -27,7 +27,6 @@ def test_default_openai_chat_request_settings():
     assert settings.number_of_responses == 1
     assert settings.logit_bias == {}
     assert settings.messages[0]["content"] == "Assistant is a large language model."
-    assert settings.response_format == "text"
 
 
 def test_custom_openai_chat_request_settings():