Fixes #20638: Document bulk create support in OpenAPI schema (#20777)

* Fixes #20638: Document bulk create support in OpenAPI schema

POST operations on NetBoxModelViewSet endpoints accept both single
objects and arrays, but the schema only documented single objects.
This prevented API client generators from producing correct code.

Add explicit bulk_create_enabled flag to NetBoxModelViewSet and
update schema generation to emit oneOf for these endpoints.

* Address PR feedback

- Removed brittle serializer marking mechanism in favor of direct checks
  on behavior.
- Attempted to introduce a bulk_create action and then route to it on
  POST in NetBoxRouter, but ran in to several obstacles including
  breaking HTTP status code reporting in the schema. Opted to simply

* Remove unused bulk_create_enabled attr

Co-authored-by: Jeremy Stretch <jstretch@netboxlabs.com>

---------

Co-authored-by: Jeremy Stretch <jstretch@netboxlabs.com>

netbox/core/api/schema.py

@@ -12,6 +12,7 @@ from drf_spectacular.utils import Direction
 
 from netbox.api.fields import ChoiceField
 from netbox.api.serializers import WritableNestedSerializer
+from netbox.api.viewsets import NetBoxModelViewSet
 
 # see netbox.api.routers.NetBoxRouter
 BULK_ACTIONS = ("bulk_destroy", "bulk_partial_update", "bulk_update")
@@ -49,6 +50,11 @@ class ChoiceFieldFix(OpenApiSerializerFieldExtension):
             )
 
 
+def viewset_handles_bulk_create(view):
+    """Check if view automatically provides list-based bulk create"""
+    return isinstance(view, NetBoxModelViewSet)
+
+
 class NetBoxAutoSchema(AutoSchema):
     """
     Overrides to drf_spectacular.openapi.AutoSchema to fix following issues:
@@ -128,6 +134,36 @@ class NetBoxAutoSchema(AutoSchema):
 
         return response_serializers
 
+    def _get_request_for_media_type(self, serializer, direction='request'):
+        """
+        Override to generate oneOf schema for serializers that support both
+        single object and array input (NetBoxModelViewSet POST operations).
+
+        Refs: #20638
+        """
+        # Get the standard schema first
+        schema, required = super()._get_request_for_media_type(serializer, direction)
+
+        # If this serializer supports arrays (marked in get_request_serializer),
+        # wrap the schema in oneOf to allow single object OR array
+        if (
+            direction == 'request' and
+            schema is not None and
+            getattr(self.view, 'action', None) == 'create' and
+            viewset_handles_bulk_create(self.view)
+        ):
+            return {
+                'oneOf': [
+                    schema,  # Single object
+                    {
+                        'type': 'array',
+                        'items': schema,  # Array of objects
+                    }
+                ]
+            }, required
+
+        return schema, required
+
     def _get_serializer_name(self, serializer, direction, bypass_extensions=False) -> str:
         name = super()._get_serializer_name(serializer, direction, bypass_extensions)