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/ExportEngineTests.cs

@@ -3,6 +3,7 @@ using System.Collections.Immutable;
 using System.IO;
 using System.Text;
 using Microsoft.Extensions.Logging.Abstractions;
+using MongoDB.Driver;
 using StellaOps.Excititor.Core;
 using StellaOps.Excititor.Export;
 using StellaOps.Excititor.Policy;
@@ -128,14 +129,14 @@ public sealed class ExportEngineTests
 
         public VexExportManifest? LastSavedManifest { get; private set; }
 
-        public ValueTask<VexExportManifest?> FindAsync(VexQuerySignature signature, VexExportFormat format, CancellationToken cancellationToken)
+        public ValueTask<VexExportManifest?> FindAsync(VexQuerySignature signature, VexExportFormat format, CancellationToken cancellationToken, IClientSessionHandle? session = null)
         {
             var key = CreateKey(signature.Value, format);
             _store.TryGetValue(key, out var manifest);
             return ValueTask.FromResult<VexExportManifest?>(manifest);
         }
 
-        public ValueTask SaveAsync(VexExportManifest manifest, CancellationToken cancellationToken)
+        public ValueTask SaveAsync(VexExportManifest manifest, CancellationToken cancellationToken, IClientSessionHandle? session = null)
         {
             var key = CreateKey(manifest.QuerySignature.Value, manifest.Format);
             _store[key] = manifest;