distribution client: return structured response for untagged and deleted models

[doringeman]

From docker/model-runner#100 (comment).
Return structured response for untagged and deleted models.

docker/model-runner#100 and docker/model-cli#122 will require small changes once this PR is merged.

Without the required changes in the model-cli, this is the response I see:

$ MODEL_RUNNER_HOST=http://localhost:8080 docker model tag ai/smollm2 dorin/smollm2
Model "ai/smollm2" tagged successfully with "index.docker.io/dorin/smollm2:latest"

$ MODEL_RUNNER_HOST=http://localhost:8080 docker model inspect ai/smollm2
{
   "id": "sha256:354bf30d0aa3af413d2aa5ae4f23c66d78980072d1e07a5b0d776e9606a2f0b9",
   "tags": [
       "ai/smollm2",
       "index.docker.io/dorin/smollm2:latest"
   ],
   "created": 1742816981,
   "config": {
       "format": "gguf",
       "quantization": "IQ2_XXS/Q4_K_M",
       "parameters": "361.82 M",
       "architecture": "llama",
       "size": "256.35 MiB"
   }
}

$ MODEL_RUNNER_HOST=http://localhost:8080 docker model rm dorin/smollm2
[{"Untagged":"index.docker.io/dorin/smollm2:latest"}]
Model dorin/smollm2 removed successfully

$ MODEL_RUNNER_HOST=http://localhost:8080 docker model tag ai/smollm2 dorin/smollm2
Model "ai/smollm2" tagged successfully with "index.docker.io/dorin/smollm2:latest"

$ MODEL_RUNNER_HOST=http://localhost:8080 docker model rm -f 354bf30d0aa3af413d2aa5ae4f23c66d78980072d1e07a5b0d776e9606a2f0b9
[{"Untagged":"ai/smollm2"},{"Untagged":"index.docker.io/dorin/smollm2:latest"},{"Deleted":"sha256:354bf30d0aa3af413d2aa5ae4f23c66d78980072d1e07a5b0d776e9606a2f0b9"}]
Model sha256:354bf30d0aa3af413d2aa5ae4f23c66d78980072d1e07a5b0d776e9606a2f0b9 removed successfully

[ilopezluna]

Oh I didn't realized in case of error we didn't return the error. Would not be better to return the error instead of empty tag/index?

[doringeman]

Yes, makes sense. In this PR or a separate one?

[ilopezluna]

I can do it in a follow up PR I just finished what I was working on so I can tackle it 👍

[doringeman]

Addressed in #109. 🙌