mlx: avoid pulling MLX models when MLX is missing (#17710)

As we look to bring Linux and Windows MLX support online, instead of blocking
downloads at the registry to avoid users wasting time downloading a model they
can't run, shift the logic to the local side which knows if MLX is present or not.

server/images.go

@@ -31,6 +31,7 @@ import (
 	"github.com/ollama/ollama/thinking"
 	"github.com/ollama/ollama/types/model"
 	"github.com/ollama/ollama/version"
+	"github.com/ollama/ollama/x/mlxrunner/mlx"
 	"github.com/ollama/ollama/x/transfer"
 )
 
@@ -1016,6 +1017,12 @@ func PullModel(ctx context.Context, name string, regOpts *registryOptions, fn fu
 	if err != nil {
 		return fmt.Errorf("pull model manifest: %s", err)
 	}
+	if hasTensorLayers(mf.Layers) {
+		if err := mlx.CheckInit(); err != nil {
+			slog.Debug("MLX is unavailable for safetensors model pull", "error", err)
+			return errors.New("this model requires MLX support, but the MLX runtime is not available")
+		}
+	}
 
 	var layers []manifest.Layer
 	layers = append(layers, mf.Layers...)