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.

docs/ARCHITECTURE_UI.md

@@ -166,6 +166,28 @@ export interface VexConsensus {
 }
 ```
 
+*Upcoming:* `NotifyApi` consumes delivery history using the new paginated envelope returned by `/api/v1/notify/deliveries`.
+
+```ts
+export interface NotifyDeliveryListResponse {
+  items: NotifyDelivery[];
+  count: number;
+  continuationToken?: string;
+}
+
+export interface NotifyDelivery {
+  deliveryId: string;
+  ruleId: string;
+  actionId: string;
+  status: 'pending'|'sent'|'failed'|'throttled'|'digested'|'dropped';
+  rendered: NotifyDeliveryRendered;
+  metadata: Record<string, string>; // includes channelType, target, previewProvider, traceId, and provider-specific entries
+  createdAt: string;
+  sentAt?: string;
+  completedAt?: string;
+}
+```
+
 ---
 
 ## 6) State, caching & real‑time