[PM-19659] Clean up Notifications code (#6244)

* Move PushType to Platform Folder

- Move the PushType next to the rest of push notification code
- Specifically exclude it from needing Platform code review
- Add tests establishing rules Platform has for usage of this enum, making it safe to have no owner

* Move NotificationHub code into Platform/Push directory

* Update NotificationHub namespace imports

* Add attribute for storing push type metadata

* Rename Push Engines to have PushEngine suffix

* Move Push Registration items to their own directory

* Push code move

* Add expected usage comment

* Add Push feature registration method

- Make method able to be called multipes times with no ill effects

* Add Push Registration service entrypoint and tests

* Use new service entrypoints

* Test changes

test/Core.Test/Platform/Push/MultiServicePushNotificationServiceTests.cs

@@ -0,0 +1,57 @@
+using Bit.Core.Enums;
+using Bit.Core.Platform.Push;
+using Bit.Core.Platform.Push.Internal;
+using Bit.Core.Settings;
+using Microsoft.Extensions.Logging.Abstractions;
+using Microsoft.Extensions.Time.Testing;
+using NSubstitute;
+using Xunit;
+
+namespace Bit.Core.Test.Platform.Push;
+
+public class MultiServicePushNotificationServiceTests
+{
+    private readonly IPushEngine _fakeEngine1;
+    private readonly IPushEngine _fakeEngine2;
+
+    private readonly MultiServicePushNotificationService _sut;
+
+    public MultiServicePushNotificationServiceTests()
+    {
+        _fakeEngine1 = Substitute.For<IPushEngine>();
+        _fakeEngine2 = Substitute.For<IPushEngine>();
+
+        _sut = new MultiServicePushNotificationService(
+            [_fakeEngine1, _fakeEngine2],
+            NullLogger<MultiServicePushNotificationService>.Instance,
+            new GlobalSettings(),
+            new FakeTimeProvider()
+        );
+    }
+
+#if DEBUG // This test requires debug code in the sut to work properly
+    [Fact]
+    public async Task PushAsync_CallsAllEngines()
+    {
+        var notification = new PushNotification<object>
+        {
+            Target = NotificationTarget.User,
+            TargetId = Guid.NewGuid(),
+            Type = PushType.AuthRequest,
+            Payload = new { },
+            ExcludeCurrentContext = false,
+        };
+
+        await _sut.PushAsync(notification);
+
+        await _fakeEngine1
+            .Received(1)
+            .PushAsync(Arg.Is<PushNotification<object>>(n => ReferenceEquals(n, notification)));
+
+        await _fakeEngine2
+            .Received(1)
+            .PushAsync(Arg.Is<PushNotification<object>>(n => ReferenceEquals(n, notification)));
+    }
+
+#endif
+}