Python: Don't return code output via on_intermediate_msg callback (#13058)

### Motivation and Context

Current, while using an OpenAIAssistantAgent, we are returning code
output via the on_intermediate_message callback. This shouldn't be
happening right now, as all other agents return code via TextContent
with a metadata `{"code": True}` key-value pair in the dictionary.

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### Description

- Closes #12956

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### Contribution Checklist

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- [X] The code builds clean without any errors or warnings
- [X] The PR follows the [SK Contribution
Guidelines](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md)
and the [pre-submission formatting
script](https://github.com/microsoft/semantic-kernel/blob/main/CONTRIBUTING.md#development-scripts)
raises no violations
- [X] All unit tests pass, and I have added new tests where possible
- [X] I didn't break anyone 😄

Author: moonbox3

python/semantic_kernel/agents/open_ai/assistant_thread_actions.py

@@ -483,7 +483,7 @@ async def invoke_stream(
                                     tool_content = generate_streaming_code_interpreter_content(agent.name, step_details)
                                     content_is_visible = True
                                 if tool_content:
-                                    if output_messages is not None:
+                                    if output_messages is not None and not content_is_visible:
                                         output_messages.append(tool_content)
                                     if content_is_visible:
                                         yield tool_content