fix(llama-cpp): keep an existing image data URL
append_media_parts always prepended data:image/jpeg;base64,.
A part that already started with data: became a doubled prefix
and the image failed to load. Leave an existing data: URL unchanged.
Assisted-by: Grok:grok-4.7
Signed-off-by: Igor Savitskiy <ils2000@mail.ru>
Co-authored-by: Ettore Di Giacinto <mudler@users.noreply.github.com>