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.

deploy/compose/mirror-gateway/conf.d/mirror-locations.conf

@@ -0,0 +1,44 @@
+proxy_set_header Host              $host;
+proxy_set_header X-Real-IP         $remote_addr;
+proxy_set_header X-Forwarded-For   $proxy_add_x_forwarded_for;
+proxy_set_header X-Forwarded-Proto $scheme;
+proxy_redirect                     off;
+
+add_header X-Cache-Status $upstream_cache_status always;
+
+location = /healthz {
+    default_type application/json;
+    return 200 '{"status":"ok"}';
+}
+
+location /concelier/exports/ {
+    proxy_pass http://concelier_backend/concelier/exports/;
+    proxy_cache mirror_cache;
+    proxy_cache_key $mirror_cache_key;
+    proxy_cache_valid 200 5m;
+    proxy_cache_valid 404 1m;
+    add_header Cache-Control "public, max-age=300, immutable" always;
+}
+
+location /concelier/ {
+    proxy_pass http://concelier_backend/concelier/;
+    proxy_cache off;
+}
+
+location /excititor/mirror/ {
+    proxy_pass http://excititor_backend/excititor/mirror/;
+    proxy_cache mirror_cache;
+    proxy_cache_key $mirror_cache_key;
+    proxy_cache_valid 200 5m;
+    proxy_cache_valid 404 1m;
+    add_header Cache-Control "public, max-age=300, immutable" always;
+}
+
+location /excititor/ {
+    proxy_pass http://excititor_backend/excititor/;
+    proxy_cache off;
+}
+
+location / {
+    return 404;
+}