perf: avoid duplicate provider metadata fetch on stop

The previous implementation fetched the provider binary path and
metadata twice per service during `compose stop`: once in stop.go
to gate on the `stop` capability, and again inside runPlugin via
setupPluginCommand.

setupPluginCommand now signals "skip" by returning (nil, nil) when
the requested command is absent from the provider's metadata.
stop.go calls runPlugin directly; the skip-when-unadvertised check
moves into runPlugin.

Addresses PR #13779 review feedback.

Signed-off-by: Guillaume Lours <glours@users.noreply.github.com>