✅(backend) add tests for document import feature

Added comprehensive tests covering DocSpec converter service,
converter orchestration, and document creation with file uploads.

Tests validate DOCX and Markdown conversion workflows, error
handling, service availability, and edge cases including empty
files and Unicode filenames.

Signed-off-by: Stephan Meijer <me@stephanmeijer.com>

src/backend/core/api/viewsets.py

@@ -43,15 +43,19 @@ from rest_framework.permissions import AllowAny
 
 from core import authentication, choices, enums, models
 from core.api.filters import remove_accents
+from core.services import mime_types
 from core.services.ai_services import AIService
 from core.services.collaboration_services import CollaborationService
 from core.services.converter_services import (
     ConversionError,
-    ServiceUnavailableError as YProviderServiceUnavailableError,
-    ValidationError as YProviderValidationError,
     Converter,
 )
-from core.services import mime_types
+from core.services.converter_services import (
+    ServiceUnavailableError as YProviderServiceUnavailableError,
+)
+from core.services.converter_services import (
+    ValidationError as YProviderValidationError,
+)
 from core.services.search_indexers import (
     get_document_indexer,
     get_visited_document_ids_of,
@@ -538,7 +542,7 @@ class DocumentViewSet(
                 converted_content = converter.convert(
                     file_content,
                     content_type=uploaded_file.content_type,
-                    accept=mime_types.YJS
+                    accept=mime_types.YJS,
                 )
                 serializer.validated_data["content"] = converted_content
                 serializer.validated_data["title"] = uploaded_file.name