🐛(backend) validate user search input data

Only the input data min length was checked. We also have to check the
mex length because the levenshtein dos not accept more than 254
characters and the email field has a max length of 254

src/backend/core/api/filters.py

@@ -128,3 +128,11 @@ class ListDocumentFilter(DocumentFilter):
 
         queryset_method = queryset.filter if bool(value) else queryset.exclude
         return queryset_method(link_traces__user=user, link_traces__is_masked=True)
+
+
+class UserSearchFilter(django_filters.FilterSet):
+    """
+    Custom filter for searching users.
+    """
+
+    q = django_filters.CharFilter(min_length=5, max_length=254)

src/backend/core/api/viewsets.py

@@ -51,7 +51,7 @@ from core.tasks.mail import send_ask_for_access_mail
 from core.utils import extract_attachments, filter_descendants
 
 from . import permissions, serializers, utils
-from .filters import DocumentFilter, ListDocumentFilter
+from .filters import DocumentFilter, ListDocumentFilter, UserSearchFilter
 from .throttling import UserListThrottleBurst, UserListThrottleSustained
 
 logger = logging.getLogger(__name__)
@@ -176,12 +176,18 @@ class UserViewSet(
         if self.action != "list":
             return queryset
 
+        filterset = UserSearchFilter(
+            self.request.GET, queryset=queryset, request=self.request
+        )
+        if not filterset.is_valid():
+            raise drf.exceptions.ValidationError(filterset.errors)
+
         # Exclude all users already in the given document
         if document_id := self.request.query_params.get("document_id", ""):
             queryset = queryset.exclude(documentaccess__document_id=document_id)
 
-        if not (query := self.request.query_params.get("q", "")) or len(query) < 5:
-            return queryset.none()
+        filter_data = filterset.form.cleaned_data
+        query = filter_data["q"]
 
         # For emails, match emails by Levenstein distance to prevent typing errors
         if "@" in query: