Python: [Feature]: Support ChatKit structured input conversion #8841

Description

agent-framework-chatkit recognizes StructuredInputItem in ThreadItemConverter, but the dispatch path currently drops it with return [] and leaves a TODO for future handling. As a result, structured input status and user answers are not included when prior ChatKit thread items are converted back into model input.

The expected behavior is to preserve the structured input as user context, including the overall status and each question as answered, skipped, or unanswered. This matches the default conversion semantics in openai-chatkit.

I am preparing a narrow implementation limited to the ChatKit converter and focused tests.