Fallback to main --model for image generation if --image-model not set

If --image-model is not specified, try to use the main --model as
the image model fallback when requesting 'default' model.

coderai

@@ -3336,11 +3336,22 @@ async def create_image_generation(request: ImageGenerationRequest):
     async with semaphore:
         image_model = multi_model_manager.image_model
     
-    # If no image model configured, return an error
+    # If no image model configured, try to use main --model as fallback
+    if not image_model:
+        # Try to get the main model from args
+        main_model = getattr(global_args, 'model', None)
+        if main_model and isinstance(main_model, list) and len(main_model) > 0:
+            image_model = main_model[0]
+            print(f"Using main --model as image model fallback: {image_model}")
+        elif main_model:
+            image_model = main_model
+            print(f"Using main --model as image model fallback: {image_model}")
+    
+    # If still no image model configured, return an error
     if not image_model:
         raise HTTPException(
             status_code=400,
-            detail="Image generation not configured. Use --image-model to specify a model."
+            detail="Image generation not configured. Use --image-model or --model to specify a model."
         )
     
     # Determine model to use