Allow tag with implicit registry

[ekcasey]

Description:

• When the <target> in docker model tag <source> <target> does not explicitly specify the registry, do not add the implicit index.docker.io to the repo name sent to the registry. Instead, allow the registry to remain implicit.
• Allows the CLI to decide what message is printed on success instead of printing whatever is sent from the backend.

Example:

> docker model tag ai/smollm2 emilycasey003/smollm2
Model "ai/smollm2" tagged successfully with "emilycasey003/smollm2"

> docker model ls
MODEL NAME                         PARAMETERS  QUANTIZATION    ARCHITECTURE  MODEL ID      CREATED       SIZE
ai/smollm2                         361.82 M    IQ2_XXS/Q4_K_M  llama         354bf30d0aa3  3 months ago  256.35 MiB
emilycasey003/smollm2:latest       361.82 M    IQ2_XXS/Q4_K_M  llama         354bf30d0aa3  3 months ago  256.35 MiB

Note: while we likely also want to do this with the implicit :latest tag so it doesn't show up explicitly in the model list when it wasn't specified, this would require the model-runner API to accept a missing tag query param and any version of the CLI that depended on this change would be incompatible with older version of model runner.

[doringeman]

Nice, thanks!