feat(agui): Add multimodal input support (image/video/audio) to AguiM…

[NoiAI]

…essage and AguiMessageConverter

Adds support for multimodal input (image, video, audio, document) in the AG-UI extension, aligning with the AG-UI Protocol InputContent specification.

• AguiMessage.content: String -> Object (backward compatible)
• AguiMessageConverter: InputContent[] -> ContentBlock conversion
• Supports both url and data (base64) source types

AgentScope-Java Version

[The version of AgentScope-Java you are working on, e.g. 1.0.12, check your pom.xml dependency version or run mvn dependency:tree | grep agentscope-parent:pom(only mac/linux)]

Description

[Please describe the background, purpose, changes made, and how to test this PR]

Checklist

Please check the following items before code is ready to be reviewed.

• Code has been formatted with mvn spotless:apply
• All tests are passing (mvn test)
• Javadoc comments are complete and follow project conventions
• Related documentation has been updated (e.g. links, examples, etc.)
• Code is ready for review

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[codecov]

Codecov Report

❌ Patch coverage is 84.61538% with 8 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
...cope/core/agui/converter/AguiMessageConverter.java	83.67%	3 Missing and 5 partials ⚠️

📢 Thoughts on this report? Let us know!

[zhengxudong]

这个功能对我们项目非常重要，我们已经测试过可以正常使用，希望能尽快合并，非常感谢！