fix(acestep-cpp): resolve relative model paths in options

[localai-bot]

Fix for issue #8991

The acestep-cpp backend was failing to load models because the model paths in options (text_encoder_model, dit_model, vae_model) were being passed to the C++ code without resolving their relative paths.

Problem

When a user configures acestep-cpp-turbo-4b, the model paths are specified as relative paths like acestep-cpp/acestep-v15-turbo-Q8_0.gguf. The backend was passing these paths directly to the C++ code without joining them with the model directory, resulting in the error:

FATAL: cannot load acestep-cpp/acestep-v15-turbo-Q8_0.gguf

Solution

This fix:

1. Gets the base directory from the ModelFile path
2. Resolves all relative paths in options to be absolute paths using filepath.Join()
3. Adds debug logging to show resolved paths for troubleshooting

Testing

The fix follows the same pattern used in other backends (e.g., llama.cpp backend) for resolving relative paths.

Fixes #8991

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[localai-bot]

I've added a DCO signoff to the commit to resolve the failing DCO check. The commit has been force-pushed to the branch. The CI should rerun and the DCO check should pass now.

[mudler]

you have now to adapt the tests of acestep, specifically TestLoadModel and TestSoundGeneration should not join anymore the modeldir:

e.g.

	Options: []string{
		"text_encoder_model:Qwen3-Embedding-0.6B-Q8_0.gguf",
		"dit_model:acestep-v15-turbo-Q8_0.gguf"),
		"vae_model:vae-BF16.gguf"),
	},

[localai-bot]

Fixed the acestep tests according to review feedback. The Options array in TestLoadModel and TestSoundGeneration no longer uses filepath.Join with modelDir. The model filenames are now specified directly without the model directory path prefix.

Changes made:

• Removed filepath.Join from the Options array in both TestLoadModel and TestSoundGeneration
• Options now contain just the model filenames (e.g., 'text_encoder_model:Qwen3-Embedding-0.6B-Q8_0.gguf')

The CI tests should now pass as the backend handles the model paths internally.