Add channel test providers for Email, Slack, Teams, and Webhook

- Implemented EmailChannelTestProvider to generate email preview payloads.
- Implemented SlackChannelTestProvider to create Slack message previews.
- Implemented TeamsChannelTestProvider for generating Teams Adaptive Card previews.
- Implemented WebhookChannelTestProvider to create webhook payloads.
- Added INotifyChannelTestProvider interface for channel-specific preview generation.
- Created ChannelTestPreviewContracts for request and response models.
- Developed NotifyChannelTestService to handle test send requests and generate previews.
- Added rate limit policies for test sends and delivery history.
- Implemented unit tests for service registration and binding.
- Updated project files to include necessary dependencies and configurations.

src/StellaOps.Excititor.WebService.Tests/MirrorEndpointsTests.cs

@@ -150,13 +150,13 @@ public sealed class MirrorEndpointsTests : IClassFixture<WebApplicationFactory<P
             FakeArtifactStore.Seed(manifest.Artifact, "{\"status\":\"ok\"}");
         }
 
-        public ValueTask<VexExportManifest?> FindAsync(VexQuerySignature signature, VexExportFormat format, CancellationToken cancellationToken)
+        public ValueTask<VexExportManifest?> FindAsync(VexQuerySignature signature, VexExportFormat format, CancellationToken cancellationToken, IClientSessionHandle? session = null)
         {
             _manifests.TryGetValue((signature.Value, format), out var manifest);
             return ValueTask.FromResult(manifest);
         }
 
-        public ValueTask SaveAsync(VexExportManifest manifest, CancellationToken cancellationToken)
+        public ValueTask SaveAsync(VexExportManifest manifest, CancellationToken cancellationToken, IClientSessionHandle? session = null)
             => ValueTask.CompletedTask;
     }