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.

tests/test_webhooks.py

@@ -0,0 +1,58 @@
+from easybill_client.webhooks import EasybillWebhookParser
+
+
+def test_parser_reads_headers_and_payload():
+    parser = EasybillWebhookParser(secret="topsecret")
+
+    payload = {"id": 42, "number": "RE-42"}
+    headers = {
+        "X-easybill-Event": "document.completed",
+        "X-easybill-Webhook-Delivery": "4711",
+    }
+
+    envelope = parser.parse_json(headers=headers, payload=payload)
+
+    assert envelope.event == "document.completed"
+    assert envelope.delivery_id == "4711"
+    assert envelope.resource_id == 42
+    assert envelope.payload == payload
+
+
+def test_parser_supports_form_encoded_payload():
+    parser = EasybillWebhookParser()
+
+    headers = {
+        "X-easybill-Event": "customer.update",
+        "X-easybill-Webhook-Delivery": "99",
+    }
+    form_data = {
+        "id": "15",
+        "customer_id": "15",
+        "company_name": "ACME GmbH",
+    }
+
+    envelope = parser.parse_form(headers=headers, form_data=form_data)
+
+    assert envelope.event == "customer.update"
+    assert envelope.delivery_id == "99"
+    assert envelope.resource_id == 15
+    assert envelope.payload["company_name"] == "ACME GmbH"
+
+
+def test_generic_parse_and_acknowledgement():
+    parser = EasybillWebhookParser()
+
+    headers = {
+        "X-easybill-Event": "document.completed",
+        "X-easybill-Webhook-Delivery": "123",
+    }
+
+    envelope = parser.parse(
+        headers=headers,
+        payload={"id": 7, "status": "done"},
+        content_type="application/json",
+    )
+
+    assert envelope.event == "document.completed"
+    assert envelope.resource_id == 7
+    assert parser.build_acknowledgement() == {"status": "ok"}