model-conversion : add causal-convert-mmproj target [no ci] (#22969)

* model-conversion : add causal-convert-mmproj target [no ci]

This commit adds a new Make target that only converts the mmproj model.

The motivation for this that the causal-convert-mm-model target will
convert both the test model and the mmproj model which is nice when the
model model conversion is finalized. But during development it is nice
to be able to just convert the mmproj model and not have to wait for
the often more time consuming text model conversion.

* add path model path validation check