Python: support ChatKit generated image conversion

[quifox (quifox)]

Motivation & Context

agent-framework-chatkit recognizes GeneratedImageItem, but the converter currently drops it with return []. This means generated images in ChatKit thread history are not provided back to the model on later turns.

Description & Review Guide

• What are the major changes? Convert completed GeneratedImageItem instances into a user Message containing a short text preface and the generated image URI. Items without an image continue to be skipped. Expose an async generated_image_to_input() override hook and add coverage for data URI media types, external URLs, missing images, custom output, ordering, failures, and cancellation.
• What is the impact of these changes? Generated images remain available as model context on later turns instead of being silently discarded. The default conversion mirrors openai-chatkit behavior. Content.from_uri() preserves embedded data URI media types; external URLs retain image/*. The async hook supports a Message, a list of messages, or None so applications can customize image context.
• What do you want reviewers to focus on? Review MIME preservation and the async customization contract. Streaming output and attachment conversion remain outside this change.

Related Issue

Fixes #8842

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 stated behavior and covers both conversion and missing-image handling.

Review effort: Balanced
Findings: None

What changed in this PR

Adds ChatKit generated images to subsequent model context.

Changes:

• Converts completed generated images into multimodal user messages.
• Continues skipping items without images.
• Adds focused conversion and dispatch coverage.

File	Description
python/​packages/​chatkit/​agent_framework_chatkit/​_converter.py	Converts generated image URLs into image content.
python/​packages/​chatkit/​tests/​test_converter.py	Verifies generated-image conversion behavior.

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