Telegram: send image output back to users

[vaayne]

Summary

When tools return image content (e.g., image generation tools like DALL-E, Stable Diffusion), the generated images should be sent back to the Telegram user as photos.

Current State

The ImageEvent plumbing exists in the stream pipeline (added in #29), but nothing produces image events because:

1. tool.Tool.Execute() returns string only
2. engine.ToolFunc returns aitypes.TextContent only
3. GoRunner.buildToolSet() wraps tool results as TextContent

Proposed Changes

1. Change engine.ToolFunc to return []aitypes.ContentBlock instead of TextContent
2. Update tool_execution.go to pass content blocks through directly
3. Update GoRunner.buildToolSet() to wrap text tools into []ContentBlock
4. In convertLoopEvent for ToolFinished, emit ImageEvent for ImageContent blocks
5. Optionally add a MultimodalTool interface for tools that return []ContentBlock

The Telegram side already handles ImageEvent — it collects images during streaming and sends them as tele.Photo after the text response.

Related

Follow-up from #29 (image input support).