Bug: Check whether model_desc.root is a dir or a zip before extraction (#719)

On bioimageio CI, the models are loaded from a folder and not a `zip`
file. So we need to check whether the `model_desc.root` is a *directory*
or a *zip file* before trying to extract it
[here](https://github.com/CAREamics/careamics/blob/d5062351b6c222177662e9329fbdce365509c5ef/src/careamics/model_io/bioimage/model_description.py#L326C5-L326C41):
```python
# extract the zip model and return the directory
model_dir = extract(model_desc.root)
```

## Changes Made
Checking the `model_desc.root` if it is a directory or a zip file:
```python
# get the model directory
if isinstance(model_desc.root, Path) and model_desc.root.is_dir():
    model_dir: DirectoryPath = model_desc.root
else:
    # extract the zip model
    model_dir = extract(model_desc.root)
```

### Modified features or files
- `model_description.py`

## How has this been tested?
I tested it by running our bmz tests as well as running the bioimageio
collection script:
[scripts/check_compatibility_careamics.py](https://github.com/bioimage-io/collection/blob/main/scripts/check_compatibility_careamics.py)


## Related Issues

<!-- Link to any related issues or discussions. Use keywords like
"Fixes", "Resolves",
or "Closes" to link to issues automatically. -->

- Resolves #712

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

Author: mese79

src/careamics/model_io/bioimage/model_description.py

@@ -26,6 +26,7 @@
     Version,
     WeightsDescr,
 )
+from pydantic import DirectoryPath
 
 from careamics.config import Configuration, DataConfig
 
@@ -322,8 +323,12 @@ def extract_model_path(model_desc: ModelDescr) -> tuple[Path, Path]:
     if model_desc.weights.pytorch_state_dict is None:
         raise ValueError("No model weights found in model description.")
 
-    # extract the zip model and return the directory
-    model_dir = extract(model_desc.root)
+    # get the model directory
+    if isinstance(model_desc.root, Path) and model_desc.root.is_dir():
+        model_dir: DirectoryPath = model_desc.root
+    else:
+        # extract the zip model
+        model_dir = extract(model_desc.root)
 
     weights_path = model_dir.joinpath(model_desc.weights.pytorch_state_dict.source.path)