agent: Add image generation appfunction

[calebareeveso]

What I have done and why

This PR adds an image generation AppFunction to the Agent demo app so the agent can generate images based on user requests.

Key Changes

• Image Generation Function (generateImage): Added a new built in AppFunction that calls the Gemini API (gemini-3.1-flash-image-preview ) to generate images from user prompts.
• Secure Image Sharing (FileProvider): Configured FileProvider ( ${applicationId}.fileprovider ) to save generated images safely to local disk and share them using secure content:// URIs.
• General Media Attachments (MessageAttachment ): Updated Room database ( AppDatabase v3) and MessageEntity to store structured media attachments with exact MIME types.
• Chat UI Card Rendering: Updated AgentDemoScreen so generated images render as distinct image cards below the message text bubble instead of relying on raw JSON output or Markdown string injection.
• Unit Tests: Added unit tests for database attachment serialization (MessageAttachmentConverterTest) and orchestrator attachment extraction (AgentOrchestratorTest )

[gemini-code-assist]

Code Review

This pull request introduces image generation capabilities to the AI assistant. It adds a new generateImage built-in AppFunction that integrates with the Gemini API, configures a FileProvider to cache and serve generated images, updates the database schema to support message attachments, and enhances the UI to display these attachments. Feedback on the changes highlights two critical issues: first, the aspectRatio parameter in the image generation function is currently ignored in the API payload; second, passing the generated content:// URIs to other apps without explicitly granting read permissions will result in a SecurityException.

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