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.conf

@@ -0,0 +1,51 @@
+proxy_cache_path /var/cache/nginx/mirror levels=1:2 keys_zone=mirror_cache:100m max_size=10g inactive=12h use_temp_path=off;
+
+map $request_uri $mirror_cache_key {
+    default $scheme$request_method$host$request_uri;
+}
+
+upstream concelier_backend {
+    server concelier:8445;
+    keepalive 32;
+}
+
+upstream excititor_backend {
+    server excititor:8448;
+    keepalive 32;
+}
+
+server {
+    listen 80;
+    server_name _;
+    return 301 https://$host$request_uri;
+}
+
+server {
+    listen 443 ssl http2;
+    server_name mirror-primary.stella-ops.org;
+
+    ssl_certificate     /etc/nginx/tls/mirror-primary.crt;
+    ssl_certificate_key /etc/nginx/tls/mirror-primary.key;
+    ssl_protocols       TLSv1.2 TLSv1.3;
+    ssl_prefer_server_ciphers on;
+
+    auth_basic "StellaOps Mirror – primary";
+    auth_basic_user_file /etc/nginx/secrets/mirror-primary.htpasswd;
+
+    include /etc/nginx/conf.d/mirror-locations.conf;
+}
+
+server {
+    listen 443 ssl http2;
+    server_name mirror-community.stella-ops.org;
+
+    ssl_certificate     /etc/nginx/tls/mirror-community.crt;
+    ssl_certificate_key /etc/nginx/tls/mirror-community.key;
+    ssl_protocols       TLSv1.2 TLSv1.3;
+    ssl_prefer_server_ciphers on;
+
+    auth_basic "StellaOps Mirror – community";
+    auth_basic_user_file /etc/nginx/secrets/mirror-community.htpasswd;
+
+    include /etc/nginx/conf.d/mirror-locations.conf;
+}