🐛(backend) fix issues with conversion microservice integration

Minor adjustments were needed after working in parallel on two PRs.
The microservice now accepts an API key without requiring it as a Bearer token.

A mistake in reading the microservice response was corrected after refactoring
the serializer to delegate logic to the converter microservice.

src/backend/core/tests/documents/test_api_documents_create_for_owner.py

@@ -25,7 +25,7 @@ def mock_convert_markdown():
     with patch.object(
         YdocConverter,
         "convert_markdown",
-        return_value={"content": "Converted document content"},
+        return_value="Converted document content",
     ) as mock:
         yield mock
 

src/backend/core/tests/test_services_converter_services.py

@@ -18,7 +18,7 @@ def test_auth_header(settings):
     """Test authentication header generation."""
     settings.CONVERSION_API_KEY = "test-key"
     converter = YdocConverter()
-    assert converter.auth_header == "Bearer test-key"
+    assert converter.auth_header == "test-key"
 
 
 def test_convert_markdown_empty_text():
@@ -116,10 +116,11 @@ def test_convert_markdown_full_integration(mock_post, settings):
         "http://test.com",
         json={"content": "test markdown"},
         headers={
-            "Authorization": "Bearer test-key",
+            "Authorization": "test-key",
             "Content-Type": "application/json",
         },
         timeout=5,
+        verify=False,
     )