Set backend for DDUF

[ilopezluna]

Follow up of #793 This pull request enhances the backend selection logic in the inference scheduler to add support for DDUF/diffusers model formats.

curl http://localhost:13434/v1/images/generations \
  -X POST \
  -H "Content-Type: application/json" \
  -d '{
    "model": "stable-diffusion",
    "prompt": "a dragon wearing a business suit",
    "negative_prompt": "blurry, low quality, distorted, watermark",
    "size": "768x512",
    "n": 2,
    "num_inference_steps": 50,
    "guidance_scale": 7.5,
    "seed": 40,
    "response_format": "b64_json"
  }'| jq -r '.data[0].b64_json' | base64 -d > dragon.png

[gemini-code-assist]

Code Review

This pull request introduces support for the diffusers backend, allowing the scheduler to handle models in DDUF and legacy diffusers formats. The backend selection logic has been refactored into a switch statement, and corresponding platform support checks and unit tests have been added. A critical issue was identified regarding a potential nil pointer dereference when logging warnings for incompatible backends, as the backend object could be nil in certain scenarios.

[sourcery-ai]

Hey - I've left some high level feedback:

• The selectBackendForModel branches for safetensors and DDUF/diffusers duplicate the pattern of computing backendName and logging a warning; consider extracting this into a small helper to keep the function shorter and reduce repetition.
• In select_backend_test.go, the tests hardcode the "llamacpp" string for backend keys and expected backend names; using llamacpp.Name instead would avoid drift if the backend name ever changes.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The `selectBackendForModel` branches for safetensors and DDUF/diffusers duplicate the pattern of computing `backendName` and logging a warning; consider extracting this into a small helper to keep the function shorter and reduce repetition.
- In `select_backend_test.go`, the tests hardcode the "llamacpp" string for backend keys and expected backend names; using `llamacpp.Name` instead would avoid drift if the backend name ever changes.

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