fix: add media/image support in Spring AI MessageConverter

[prasadskarmarkar]

Previously, MessageConverter only transferred text content from ADK to Spring AI, ignoring image and media attachments. This caused vision model requests to fail even though Spring AI's underlying models (like GPT-4o) support image inputs.

Updated MessageConverter to properly handle image/media parts by constructing UserMessage with Media attachments.

Fixes #705

[gemini-code-assist]

Summary of Changes

Hello @prasadskarmarkar, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request introduces crucial enhancements to the Spring AI MessageConverter by enabling full support for image and media attachments. Previously, the converter was limited to text-only content, which prevented the successful execution of vision model requests. The updated logic now ensures that UserMessage objects are correctly built with media parts, resolving issue #705 and allowing for richer, multimodal interactions with AI models.

Highlights

• Enhanced Media Support: The MessageConverter now correctly processes image and media attachments from the ADK, enabling vision model requests that previously failed due to ignored media content.
• Conditional UserMessage Construction: The handleUserContent method has been updated to conditionally construct UserMessage objects, including media attachments when present, using UserMessage.builder().media(mediaList).build().
• Comprehensive Test Coverage: New unit tests have been added to MessageConverterTest to validate the proper handling of inline media, file-based media, multiple media attachments, and messages containing only media.

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[gemini-code-assist]

Code Review

This pull request effectively adds support for media and image attachments to the MessageConverter, which was a missing feature. The implementation is straightforward and correct. The addition of comprehensive unit tests covering various scenarios (inline data, file URI, multiple attachments, media-only messages) is excellent and ensures the new functionality is well-tested. I have a few minor suggestions to improve code conciseness and test robustness.