server : fix MTMD image_url injection

[pockers21]

Summary

• normalize chat-template vision markers <start_of_image><end_of_image> to mtmd_default_marker() when image_url content is present
• ensure process_mtmd_prompt() actually receives the downloaded image bytes so the vision response matches expectations

Testing

• DEBUG_EXTERNAL=1 LLAMA_CACHE= path_of_llama-cache
  pytest -q -x tools/server/tests/unit/test_vision_api.py::test_vision_chat_completion -k 'IMG_URL_0 or IMG_BASE64_URI_0'

[ngxson]

No, this won't work across all models. You must clean the chat template instead