Enhance output_content_type detection in AssistantAgent.

[smalltalkman]

Why are these changes needed?

Enhance output_content_type checking in AssistantAgent to prevent illegal types from being used to construct StructuredMessage.
This will also allow setting output_content_type=True on the AssistantAgent to output JSON output instead of json code blocks.

Related issue number

Closes #6727

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[ekzhu]

Thanks. Maybe we should validate the type in the constructor instead?

[smalltalkman]

Maybe we should validate the type in the constructor instead?

The current modification focuses on the compatibility of output_content_type with json_output. So that json_output=True can be passed by setting output_content_type=True.

If strong validation of type is required in the constructor, I think the constructor interface should be modified to use json_output directly (currently output_content_type is mainly used for json_output). This is beyond the scope of this PR.