fix(artifact): show :latest tag in ls output when no explicit tag given

When an artifact is added without an explicit tag (e.g.
"quay.io/myimage/myartifact"), the TAG column in "podman artifact ls"
was empty instead of showing "latest", unlike container images which
default to :latest.

Add a call to reference.TagNameOnly() after parsing the stored name so
the display normalises the reference before extracting the tag, matching
the behaviour of container images.

Fixes: #27083

Signed-off-by: Devesh B <[email protected]>