server : fix image blocks in tool_result being dropped during Anthropic OpenAI conversion (#22536)

• 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