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.Concelier.Storage.Mongo.Tests/Migrations/MongoMigrationRunnerTests.cs

@@ -215,15 +215,59 @@ public sealed class MongoMigrationRunnerTests
 
             Assert.DoesNotContain(indexList, x => x["name"].AsString == "gridfs_files_expiresAt_ttl");
         }
-        finally
-        {
-            await _fixture.Client.DropDatabaseAsync(databaseName);
-        }
-    }
-
-    private sealed class TestMigration : IMongoMigration
-    {
-        public int ApplyCount { get; private set; }
+        finally
+        {
+            await _fixture.Client.DropDatabaseAsync(databaseName);
+        }
+    }
+
+    [Fact]
+    public async Task EnsureAdvisoryEventCollectionsMigration_CreatesIndexes()
+    {
+        var databaseName = $"concelier-advisory-events-{Guid.NewGuid():N}";
+        var database = _fixture.Client.GetDatabase(databaseName);
+        await database.CreateCollectionAsync(MongoStorageDefaults.Collections.AdvisoryStatements);
+        await database.CreateCollectionAsync(MongoStorageDefaults.Collections.AdvisoryConflicts);
+        await database.CreateCollectionAsync(MongoStorageDefaults.Collections.Migrations);
+
+        try
+        {
+            var migration = new EnsureAdvisoryEventCollectionsMigration();
+            var runner = new MongoMigrationRunner(
+                database,
+                new IMongoMigration[] { migration },
+                NullLogger<MongoMigrationRunner>.Instance,
+                TimeProvider.System);
+
+            await runner.RunAsync(CancellationToken.None);
+
+            var statementIndexes = await database
+                .GetCollection<BsonDocument>(MongoStorageDefaults.Collections.AdvisoryStatements)
+                .Indexes
+                .ListAsync();
+            var statementIndexNames = (await statementIndexes.ToListAsync()).Select(x => x["name"].AsString).ToArray();
+
+            Assert.Contains("advisory_statements_vulnerability_asof_desc", statementIndexNames);
+            Assert.Contains("advisory_statements_statementHash_unique", statementIndexNames);
+
+            var conflictIndexes = await database
+                .GetCollection<BsonDocument>(MongoStorageDefaults.Collections.AdvisoryConflicts)
+                .Indexes
+                .ListAsync();
+            var conflictIndexNames = (await conflictIndexes.ToListAsync()).Select(x => x["name"].AsString).ToArray();
+
+            Assert.Contains("advisory_conflicts_vulnerability_asof_desc", conflictIndexNames);
+            Assert.Contains("advisory_conflicts_conflictHash_unique", conflictIndexNames);
+        }
+        finally
+        {
+            await _fixture.Client.DropDatabaseAsync(databaseName);
+        }
+    }
+
+    private sealed class TestMigration : IMongoMigration
+    {
+        public int ApplyCount { get; private set; }
 
         public string Id => "999_test";