feat: add placeholder support for empty model dropdowns

Add get_model_placeholder() helper function to folder_paths.py that
generates user-friendly placeholder text for empty model dropdowns.

Update model loader nodes to include placeholder in INPUT_TYPES:
- CheckpointLoaderSimple
- unCLIPCheckpointLoader
- LoraLoader
- LoraLoaderModelOnly
- VAELoader
- ControlNetLoader
- DiffControlNetLoader
- UNETLoader
- CLIPLoader
- DualCLIPLoader
- CLIPVisionLoader

The placeholder text shows 'No models found in ComfyUI/models/{folder} folder . . .'
when the options list is empty, helping users understand where to place models.

Amp-Thread-ID: https://ampcode.com/threads/T-019c2bd5-472a-73a1-842b-4e05cba5d12c

folder_paths.py

@@ -472,6 +472,18 @@ def get_save_image_path(filename_prefix: str, output_dir: str, image_width=0, im
         counter = 1
     return full_output_folder, filename, counter, subfolder, filename_prefix
 
+def get_model_placeholder(folder_name: str) -> str:
+    """Generate placeholder text for empty model dropdowns.
+
+    Args:
+        folder_name: The name of the model folder (e.g., "checkpoints", "loras").
+
+    Returns:
+        A user-friendly placeholder string indicating where models should be placed.
+    """
+    return f"No models found in ComfyUI/models/{folder_name} folder . . ."
+
+
 def get_input_subfolders() -> list[str]:
     """Returns a list of all subfolder paths in the input directory, recursively.