fix: convert image string to ImageRow object on model service creation

[lizable]

resolves #5633 (BA-MMM)

The fix addresses an issue in src/ai/backend/manager/sokovan/deployment/deployment_controller.py:517 where model service creation was failing due to improper image handling:

• Added image resolution logic: Uses ImageRow.resolve() to convert image string to proper ImageRef object
• Fixed type mismatch: kernel_config.image_ref now receives the correctly typed ImageRef instead of a raw string
• Maintained consistency: Follows the same image resolution pattern used in regular session creation
• Preserved JSON serialization: Kept endpoint_resource_opts.image_ref as string for proper serialization

The change ensures model services can be created successfully by providing the kernel configuration with the expected ImageRef object type while maintaining backward compatibility for stored endpoint resource options.

Checklist: (if applicable)

• Milestone metadata specifying the target backport version
• Mention to the original issue
• Installer updates including:
  • Fixtures for db schema changes
  • New mandatory config options
• Update of end-to-end CLI integration tests in ai.backend.test
• API server-client counterparts (e.g., manager API -> client SDK)
• Test case(s) to:
  • Demonstrate the difference of before/after
  • Demonstrate the flow of abstract/conceptual models with a concrete implementation
• Documentation
  • Contents in the docs directory
  • docstrings in public interfaces and type annotations

[Copilot]

Pull Request Overview

This PR fixes a type mismatch issue in model service creation where a string image identifier was being passed instead of the expected ImageRef object. The fix implements proper image resolution to convert the string to an ImageRef object while maintaining backward compatibility for JSON serialization.

Key changes:

• Added image resolution logic using ImageRow.resolve() to convert image string to ImageRef object
• Updated kernel_config.image_ref to use the resolved ImageRef instead of raw string
• Maintained string format for endpoint_resource_opts.image_ref to preserve JSON serialization compatibility

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[Copilot]

The ImageRow.resolve() call lacks error handling. If image resolution fails (e.g., image not found), this could raise an unhandled exception. Consider wrapping this in a try-catch block and providing a meaningful error message for debugging model service creation failures.

Suggested change

	image_row = await ImageRow.resolve(
	session,
	[
	ImageIdentifier(model_spec.image, model_spec.architecture),
	ImageAlias(model_spec.image),
	],
	)
	try:
	image_row = await ImageRow.resolve(
	session,
	[
	ImageIdentifier(model_spec.image, model_spec.architecture),
	ImageAlias(model_spec.image),
	],
	)
	except Exception as exc:
	log.error(
	"Failed to resolve image for model service creation: image='{}', architecture='{}'. Exception: {}",
	model_spec.image,
	model_spec.architecture,
	exc,
	)
	raise ServiceInfoRetrievalFailed(
	f"Failed to resolve image '{model_spec.image}' (architecture: '{model_spec.architecture}') for model service creation."
	) from exc