common: resolve draft repo to its requested sidecar (#25955)

common: resolve draft repo to its requested sidecar (#25955)

With -hfd pointing to a repo shipping speculative sidecars, the draft
resolved to the main model of that repo, since find_best_model()
excludes sidecar files, and the explicit draft plan suppressed the
sidecar discovery on the -hf repo.

The draft plan already discovers its sidecars, they were just never
consumed. Wire them as the draft, following the fallback pattern of
the main plan, so this now works as expected:

llama-server -hf repo -hfd repo --spec-type draft-dflash