Fixes#6140
When a tool returns CallToolResult with multiple content items (e.g.,
both TextContent and ImageContent), the agent was only processing
content[0], ignoring the rest.
Changes:
- Replace direct content[0] access with enumerate(res.content) loop
- Process all content items: TextContent, ImageContent, EmbeddedResource
- Use content_index for image caching to distinguish multiple images
This fixes the issue where tools like Bilibili plugin return both
text descriptions and screenshots, but LLM only received one of them.
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