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/concelier.yaml.sample

@@ -15,7 +15,7 @@ storage:
 plugins:
   # Concelier resolves plug-ins relative to the content root; override as needed.
   baseDirectory: ".."
-  directory: "PluginBinaries"
+  directory: "StellaOps.Concelier.PluginBinaries"
   searchPatterns:
     - "StellaOps.Concelier.Plugin.*.dll"
 
@@ -74,6 +74,22 @@ authority:
     - "127.0.0.1/32"
     - "::1/128"
 
+mirror:
+  enabled: false
+  # Directory containing JSON exporter outputs (absolute or relative to content root).
+  exportRoot: "exports/json"
+  # Optional explicit export identifier; defaults to `latest` symlink or most recent export.
+  activeExportId: ""
+  latestDirectoryName: "latest"
+  mirrorDirectoryName: "mirror"
+  requireAuthentication: false
+  maxIndexRequestsPerHour: 600
+  domains:
+    - id: "primary"
+      displayName: "Primary Mirror"
+      requireAuthentication: false
+      maxDownloadRequestsPerHour: 1200
+
 sources:
   ghsa:
     apiToken: "${GITHUB_PAT}"