_tool_result_block passed list content through unchanged, so image_url
blocks returned by tools (e.g. read_file on an image file, which
returns OpenAI-format image_url blocks via build_image_content_blocks)
reached the Anthropic API unconverted and were rejected. User-role
messages already ran through _convert_user_content at the call site,
so inbound Telegram photos worked, but tool results did not.
Run _convert_user_content on list content inside _tool_result_block
so image_url blocks become native Anthropic image blocks. Required
making _convert_user_content a @staticmethod (it did not use self)
and calling _convert_image_block via the class to match.
Repro: an agent calling read_file on any image file got a
"Non-transient LLM error with image content, retrying without images"
warning and the image was silently dropped from the conversation.
