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/.github/workflows/python.yml

@@ -0,0 +1,34 @@
+# NOTE: This file is auto generated by OpenAPI Generator.
+# URL: https://openapi-generator.tech
+#
+# ref: https://docs.github.com/en/actions/automating-builds-and-tests/building-and-testing-python
+
+name: easybill_generated_async Python package
+
+on: [push, pull_request]
+
+permissions:
+  contents: read
+
+jobs:
+  build:
+
+    runs-on: ubuntu-latest
+    strategy:
+      matrix:
+        python-version: ["3.10", "3.11", "3.12", "3.13", "3.14"]
+
+    steps:
+      - uses: actions/checkout@v4
+      - name: Set up Python ${{ matrix.python-version }}
+        uses: actions/setup-python@v4
+        with:
+          python-version: ${{ matrix.python-version }}
+      - name: Install dependencies
+        run: |
+          python -m pip install --upgrade pip
+          pip install -r requirements.txt
+          pip install -r test-requirements.txt
+      - name: Test with pytest
+        run: |
+          pytest --cov=easybill_generated_async