Add unit tests for authentication and webhook parsing

- Implement tests for basic and bearer authentication headers in `test_auth.py`.
- Create tests for the `EasybillWebhookParser` in `test_webhooks.py`, covering JSON and form-encoded payloads, as well as a generic parse and acknowledgement method.

generated/sync/docs/SerialNumber.md

@@ -0,0 +1,35 @@
+# SerialNumber
+
+
+## Properties
+
+Name | Type | Description | Notes
+------------ | ------------- | ------------- | -------------
+**id** | **int** |  | [optional] [readonly] 
+**serial_number** | **str** |  | 
+**position_id** | **int** |  | 
+**document_id** | **int** |  | [optional] [readonly] 
+**document_position_id** | **int** |  | [optional] [readonly] 
+**used_at** | **str** |  | [optional] [readonly] 
+**created_at** | **str** |  | [optional] [readonly] 
+
+## Example
+
+```python
+from easybill_generated_sync.models.serial_number import SerialNumber
+
+# TODO update the JSON string below
+json = "{}"
+# create an instance of SerialNumber from a JSON string
+serial_number_instance = SerialNumber.from_json(json)
+# print the JSON string representation of the object
+print(SerialNumber.to_json())
+
+# convert the object into a dict
+serial_number_dict = serial_number_instance.to_dict()
+# create an instance of SerialNumber from a dict
+serial_number_from_dict = SerialNumber.from_dict(serial_number_dict)
+```
+[[Back to Model list]](../README.md#documentation-for-models) [[Back to API list]](../README.md#documentation-for-api-endpoints) [[Back to README]](../README.md)
+
+