Adds a focused regression test so the fix for tool_result image
handling cannot silently revert. Two cases:
- list content with an image_url + text block -> image_url is
translated to a native Anthropic image block, sibling text passes
through unchanged
- plain string content passes through untouched (the new list branch
must not alter the string path)
These cover the exact symptom surface (silent image drop with a
"Non-transient LLM error with image content" warning) and the only
two content shapes tool results actually take today.
Made-with: Cursor
