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.

etc/notify.yaml.sample

@@ -24,6 +24,19 @@ api:
   basePath: "/api/v1/notify"
   internalBasePath: "/internal/notify"
   tenantHeader: "X-StellaOps-Tenant"
+  rateLimits:
+    deliveryHistory:
+      enabled: true
+      tokenLimit: 60
+      tokensPerPeriod: 30
+      replenishmentPeriodSeconds: 60
+      queueLimit: 20
+    testSend:
+      enabled: true
+      tokenLimit: 5
+      tokensPerPeriod: 5
+      replenishmentPeriodSeconds: 60
+      queueLimit: 2
 
 plugins:
   baseDirectory: "../"