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.Export.Tests/VexExportCacheServiceTests.cs

@@ -1,4 +1,5 @@
 using Microsoft.Extensions.Logging.Abstractions;
+using MongoDB.Driver;
 using StellaOps.Excititor.Core;
 using StellaOps.Excititor.Export;
 using StellaOps.Excititor.Storage.Mongo;
@@ -72,10 +73,10 @@ public sealed class VexExportCacheServiceTests
         public int ExpiredCount { get; set; }
         public int DanglingCount { get; set; }
 
-        public ValueTask<int> RemoveExpiredAsync(DateTimeOffset asOf, CancellationToken cancellationToken)
+        public ValueTask<int> RemoveExpiredAsync(DateTimeOffset asOf, CancellationToken cancellationToken, IClientSessionHandle? session = null)
             => ValueTask.FromResult(ExpiredCount);
 
-        public ValueTask<int> RemoveMissingManifestReferencesAsync(CancellationToken cancellationToken)
+        public ValueTask<int> RemoveMissingManifestReferencesAsync(CancellationToken cancellationToken, IClientSessionHandle? session = null)
             => ValueTask.FromResult(DanglingCount);
     }
 }