Python: support ChatKit structured input conversion

[quick_fox (quifox)]

Motivation & Context

agent-framework-chatkit recognizes StructuredInputItem, but the converter currently drops it with return []. This means structured input status and user answers are missing when ChatKit thread history is converted back into model input.

Description & Review Guide

• What are the major changes? Convert StructuredInputItem into a user Message that preserves the overall status and represents each question as answered, skipped, or unanswered. Add focused coverage for multiple answers, skipped answers, unanswered questions, and dispatch ordering.
• What is the impact of these changes? Structured input interactions remain available to the agent on later turns instead of being silently discarded. The conversion mirrors the default openai-chatkit behavior and stays inside the ChatKit adapter.
• What do you want reviewers to focus on? The mapping intentionally preserves the existing ChatKit semantics without adding a new public API.

Related Issue

Fixes #8841

Contribution Checklist

• The code builds clean without any errors or warnings
• All unit tests pass, and I have added new tests where possible
• The PR follows the Contribution Guidelines
• This PR is linked to an issue and there is no other open PR for this issue (see Related Issue above).
• This is not a breaking change. If it is a breaking change, add the breaking change label (or add "[BREAKING]" to the title prefix, before or after any language prefix) — a workflow keeps the label and title prefix in sync automatically.

[Copilot]

Copilot review overview

🟢 Approval recommended

The implementation matches the documented behavior and includes focused coverage for the supported states.

Review effort: Balanced
Findings: None

What changed in this PR

Adds ChatKit structured-input conversion so prior statuses and answers remain available as user context.

Changes:

• Converts answered, skipped, and unanswered structured inputs into a user message.
• Adds focused conversion and dispatch-order tests.

File	Description
python/​packages/​chatkit/​agent_framework_chatkit/​_converter.py	Implements structured-input conversion and dispatch.
python/​packages/​chatkit/​tests/​test_converter.py	Covers conversion semantics and ordering.

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