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/PDFTemplates.md

@@ -0,0 +1,29 @@
+# PDFTemplates
+
+
+## Properties
+
+Name | Type | Description | Notes
+------------ | ------------- | ------------- | -------------
+**items** | [**List[PDFTemplate]**](PDFTemplate.md) |  | [optional] 
+
+## Example
+
+```python
+from easybill_generated_sync.models.pdf_templates import PDFTemplates
+
+# TODO update the JSON string below
+json = "{}"
+# create an instance of PDFTemplates from a JSON string
+pdf_templates_instance = PDFTemplates.from_json(json)
+# print the JSON string representation of the object
+print(PDFTemplates.to_json())
+
+# convert the object into a dict
+pdf_templates_dict = pdf_templates_instance.to_dict()
+# create an instance of PDFTemplates from a dict
+pdf_templates_from_dict = PDFTemplates.from_dict(pdf_templates_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)
+
+