* server : accept data: URLs for input_video and input_audio
input_video and input_audio passed accept_base64_uri=false to
handle_media(), so data: URLs got treated as raw base64 strings and
failed later with a confusing media probe error (#27724).
pass true for these two content types the same way image_url already
does, and allow video/audio mime types in the data: url check instead
of image only. data URL validation now throws std::invalid_argument so
malformed input comes back as 400 instead of 500, matching the other
input validation in this file.
* server : simplify handle_media and drop unused accept_base64_uri flag
* server : update comment and add unit test for invalid data URI MIME
