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.Storage.Mongo.Tests/MongoVexRepositoryTests.cs

@@ -108,7 +108,8 @@ public sealed class MongoVexRepositoryTests : IAsyncLifetime
             GridFsInlineThresholdBytes = 64,
         });
 
-        var store = new MongoVexExportStore(_client, database, options);
+        var sessionProvider = new VexMongoSessionProvider(_client, options);
+        var store = new MongoVexExportStore(_client, database, options, sessionProvider);
         var signature = new VexQuerySignature("format=csaf|provider=redhat");
         var manifest = new VexExportManifest(
             "exports/20251016/redhat",
@@ -152,7 +153,8 @@ public sealed class MongoVexRepositoryTests : IAsyncLifetime
             GridFsInlineThresholdBytes = 64,
         });
 
-        var store = new MongoVexExportStore(_client, database, options);
+        var sessionProvider = new VexMongoSessionProvider(_client, options);
+        var store = new MongoVexExportStore(_client, database, options, sessionProvider);
         var signature = new VexQuerySignature("format=json|provider=cisco");
         var manifest = new VexExportManifest(
             "exports/20251016/cisco",
@@ -263,7 +265,8 @@ public sealed class MongoVexRepositoryTests : IAsyncLifetime
             ExportCacheTtl = TimeSpan.FromHours(1),
         });
 
-        return new MongoVexRawStore(_client, database, options);
+        var sessionProvider = new VexMongoSessionProvider(_client, options);
+        return new MongoVexRawStore(_client, database, options, sessionProvider);
     }
 
     private static string BuildExportKey(VexQuerySignature signature, VexExportFormat format)