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/README.md

@@ -0,0 +1,13 @@
+# Mirror Gateway Assets
+
+This directory holds the reverse-proxy configuration and TLS material for the managed
+mirror profile:
+
+- `conf.d/*.conf` – nginx configuration shipped with the profile.
+- `tls/` – place environment-specific certificates and private keys
+  (`mirror-primary.{crt,key}`, `mirror-community.{crt,key}`, etc.).
+- `secrets/` – populate Basic Auth credential stores (`*.htpasswd`) that gate each
+  mirror domain. Generate with `htpasswd -B`.
+
+The Compose bundle mounts these paths read-only. Populate `tls/` with the actual
+certificates before invoking `docker compose config` or `docker compose up`.