Validate collisions in domain assignments (#15759)

* Validate collisions in domain assignments

* Update OpenApi.json

src/Umbraco.Core/Models/ContentEditing/DomainUpdateResult.cs

@@ -0,0 +1,8 @@
+namespace Umbraco.Cms.Core.Models.ContentEditing;
+
+public sealed class DomainUpdateResult
+{
+    public IEnumerable<IDomain> Domains { get; init; } = Enumerable.Empty<IDomain>();
+
+    public IEnumerable<IDomain>? ConflictingDomains { get; init; }
+}