spec: fix failed to decode mtmd chunk with DFlash (#28587)
* speculative: fix failed to decode mtmd chunk with DFlash
When using DFlash w/ vision models, the drafter memory fails to
allocate new tokens because images report a fixed offset. Stop copying
them to allow the drafter to continue.
* address PR feedback
limit M-RoPE skip to images only, allow audio to pass through. Clean up
comments to align to the updated implementation
