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/notify/samples/notify-delivery-list-response.sample.json

@@ -0,0 +1,46 @@
+{
+  "items": [
+    {
+      "deliveryId": "delivery-7f3b6c51",
+      "tenantId": "tenant-acme",
+      "ruleId": "rule-critical-slack",
+      "actionId": "slack-secops",
+      "eventId": "4f6e9c09-01b4-4c2a-8a57-3d06de182d74",
+      "kind": "scanner.report.ready",
+      "status": "Sent",
+      "statusReason": null,
+      "rendered": {
+        "channelType": "Slack",
+        "format": "Slack",
+        "target": "#sec-alerts",
+        "title": "Critical findings detected",
+        "body": "{\"text\":\"Critical findings detected\",\"blocks\":[{\"type\":\"section\",\"text\":{\"type\":\"mrkdwn\",\"text\":\"*Critical findings detected*\\n1 new critical finding across 2 images.\"}},{\"type\":\"context\",\"elements\":[{\"type\":\"mrkdwn\",\"text\":\"Preview generated 2025-10-19T16:23:41.889Z · Trace `trace-58c212`\"}]}]}",
+        "summary": "1 new critical finding across 2 images.",
+        "textBody": "1 new critical finding across 2 images.\nTrace: trace-58c212",
+        "locale": "en-us",
+        "bodyHash": "febf9b2a630d862b07f4390edfbf31f5e8b836529f5232c491f4b3f6dba4a4b2",
+        "attachments": []
+      },
+      "attempts": [
+        {
+          "timestamp": "2025-10-19T16:23:42.112Z",
+          "status": "Succeeded",
+          "statusCode": 200,
+          "reason": null
+        }
+      ],
+      "metadata": {
+        "channelType": "slack",
+        "target": "#sec-alerts",
+        "previewProvider": "fallback",
+        "traceId": "trace-58c212",
+        "slack.channel": "#sec-alerts"
+      },
+      "createdAt": "2025-10-19T16:23:41.889Z",
+      "sentAt": "2025-10-19T16:23:42.101Z",
+      "completedAt": "2025-10-19T16:23:42.112Z"
+    }
+  ],
+  "count": 1,
+  "continuationToken": "2025-10-19T16:23:41.889Z|tenant-acme:delivery-7f3b6c51"
+}