Fix CI test

The test was written in a way that would work locally if you had
a specific S3 configuration, but failed in CI (or any other context with
a different set of values).

tests/test_conversions.py

@@ -20,7 +20,7 @@ def test_create_conversion_returns_result():
     assert response.json() == {
         "id": 1,
         "result_url": None,
-        "source_url": "https://s3.us-west-002.backblazeb2.com/lfc-llm-pdf-dev/my-fake-file",
+        "source_url": f"{settings.S3_ENDPOINT}/{settings.S3_BUCKET}/my-fake-file",
         "status": "pending",
     }