fix(openapi): add BindingErrorResponse schema

OAuthRegisterBadRequestResponse references BindingErrorResponse but
that schema wasn't in the original add. Adding it now as a cloud-only
schema matching the cloud runtime's binding-error shape (single
'message' string field).

openapi.yaml

@@ -9940,3 +9940,13 @@ components:
           description: When the user joined the workspace (same as created_at for the workspace creator)
         subscription_tier:
           $ref: '#/components/schemas/SubscriptionTier'
+
+    BindingErrorResponse:
+      type: object
+      x-runtime: [cloud]
+      description: '[cloud-only] Error shape returned when request binding or validation fails before the handler runs.'
+      required:
+      - message
+      properties:
+        message:
+          type: string