* server : fix image blocks in tool_result being dropped during Anthropic→OpenAI conversion
server_chat_convert_anthropic_to_oai() silently discarded image blocks
inside Anthropic tool_result content. This broke multimodal tool outputs
(e.g. a tool that returns an image) because the model never received the
image.
When tool_result contains image blocks, convert them to OpenAI
multimodal content parts (text + image_url array). Plain-text results
remain simple strings for backwards compatibility.
* server : add test for image blocks in Anthropic tool_result conversion