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_NOTIFY.md

@@ -37,6 +37,8 @@ src/
 **Dependencies**: Authority (OpToks; DPoP/mTLS), MongoDB, Redis/NATS (bus), HTTP egress to Slack/Teams/Webhooks, SMTP relay for Email.
 
 > **Configuration.** Notify.WebService bootstraps from `notify.yaml` (see `etc/notify.yaml.sample`). Use `storage.driver: mongo` with a production connection string; the optional `memory` driver exists only for tests. Authority settings follow the platform defaults—when running locally without Authority, set `authority.enabled: false` and supply `developmentSigningKey` so JWTs can be validated offline.
+>
+> `api.rateLimits` exposes token-bucket controls for delivery history queries and test-send previews (`deliveryHistory`, `testSend`). Default values allow generous browsing while preventing accidental bursts; operators can relax/tighten the buckets per deployment.
 
 > **Plug-ins.** All channel connectors are packaged under `<baseDirectory>/plugins/notify`. The ordered load list must start with Slack/Teams before Email/Webhook so chat-first actions are registered deterministically for Offline Kit bundles:
 >
@@ -204,6 +206,8 @@ public interface INotifyConnector {
 
 **DeliveryContext** includes **rendered content** and **raw event** for audit.
 
+**Test-send previews.** Plug-ins can optionally implement `INotifyChannelTestProvider` to shape `/channels/{id}/test` responses. Providers receive a sanitised `ChannelTestPreviewContext` (channel, tenant, target, timestamp, trace) and return a `NotifyDeliveryRendered` preview + metadata. When no provider is present, the host falls back to a generic preview so the endpoint always responds.
+
 **Secrets**: `ChannelConfig.secretRef` points to Authority‑managed secret handle or K8s Secret path; workers load at send-time; plug-in manifests (`notify-plugin.json`) declare capabilities and version.
 
 ---
@@ -294,7 +298,7 @@ Internal tooling can hit `/internal/notify/<entity>/normalize` to upgrade legacy
 * **Channels**
 
   * `POST /channels` | `GET /channels` | `GET /channels/{id}` | `PATCH /channels/{id}` | `DELETE /channels/{id}`
-  * `POST /channels/{id}/test` → send sample message (no rule evaluation)
+  * `POST /channels/{id}/test` → send sample message (no rule evaluation); returns `202 Accepted` with rendered preview + metadata (base keys: `channelType`, `target`, `previewProvider`, `traceId` + connector-specific entries); governed by `api.rateLimits:testSend`.
   * `GET /channels/{id}/health` → connector self‑check
 
 * **Rules**
@@ -305,7 +309,7 @@ Internal tooling can hit `/internal/notify/<entity>/normalize` to upgrade legacy
 * **Deliveries**
 
   * `POST /deliveries` → ingest worker delivery state (idempotent via `deliveryId`).
-  * `GET /deliveries?since=...&status=...&limit=...` → list (most recent first)
+  * `GET /deliveries?since=...&status=...&limit=...` → list envelope `{ items, count, continuationToken }` (most recent first); base metadata keys match the test-send response (`channelType`, `target`, `previewProvider`, `traceId`); rate-limited via `api.rateLimits.deliveryHistory`. See `docs/notify/samples/notify-delivery-list-response.sample.json`.
   * `GET /deliveries/{id}` → detail (redacted body + metadata)
   * `POST /deliveries/{id}/retry` → force retry (admin, future sprint)