✅(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/serializers.py

@@ -11,11 +11,11 @@ from django.utils.functional import lazy
 from django.utils.text import slugify
 from django.utils.translation import gettext_lazy as _
 
-from core.services import mime_types
 import magic
 from rest_framework import serializers
 
 from core import choices, enums, models, utils, validators
+from core.services import mime_types
 from core.services.ai_services import AI_ACTIONS
 from core.services.converter_services import (
     ConversionError,
@@ -441,9 +441,7 @@ class ServerCreateDocumentSerializer(serializers.Serializer):
 
         try:
             document_content = Converter().convert(
-                validated_data["content"],
-                mime_types.MARKDOWN,
-                mime_types.YJS
+                validated_data["content"], mime_types.MARKDOWN, mime_types.YJS
             )
         except ConversionError as err:
             raise serializers.ValidationError(