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/async/.gitlab-ci.yml

@@ -0,0 +1,31 @@
+# NOTE: This file is auto generated by OpenAPI Generator.
+# URL: https://openapi-generator.tech
+#
+# ref: https://docs.gitlab.com/ee/ci/README.html
+# ref: https://gitlab.com/gitlab-org/gitlab/-/blob/master/lib/gitlab/ci/templates/Python.gitlab-ci.yml
+
+stages:
+  - test
+
+.pytest:
+  stage: test
+  script:
+   - pip install -r requirements.txt
+   - pip install -r test-requirements.txt
+   - pytest --cov=easybill_generated_async
+
+pytest-3.10:
+  extends: .pytest
+  image: python:3.10-alpine
+pytest-3.11:
+  extends: .pytest
+  image: python:3.11-alpine
+pytest-3.12:
+  extends: .pytest
+  image: python:3.12-alpine
+pytest-3.13:
+  extends: .pytest
+  image: python:3.13-alpine
+pytest-3.14:
+  extends: .pytest
+  image: python:3.14-alpine