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.

docs/dev/31_AUTHORITY_PLUGIN_DEVELOPER_GUIDE.md

@@ -13,7 +13,7 @@ Authority plug-ins extend the **StellaOps Authority** service with custom identi
 Authority hosts follow a deterministic plug-in lifecycle. The exported diagram (`docs/assets/authority/authority-plugin-lifecycle.svg`) mirrors the steps below; regenerate it from the Mermaid source if you update the flow.
 
 1. **Configuration load** – `AuthorityPluginConfigurationLoader` resolves YAML manifests under `etc/authority.plugins/`.
-2. **Assembly discovery** – the shared `PluginHost` scans `PluginBinaries/Authority` for `StellaOps.Authority.Plugin.*.dll` assemblies.
+2. **Assembly discovery** – the shared `PluginHost` scans `StellaOps.Authority.PluginBinaries` for `StellaOps.Authority.Plugin.*.dll` assemblies.
 3. **Registrar execution** – each assembly is searched for `IAuthorityPluginRegistrar` implementations. Registrars bind options, register services, and optionally queue bootstrap tasks.
 4. **Runtime** – the host resolves `IIdentityProviderPlugin` instances, uses capability metadata to decide which OAuth grants to expose, and invokes health checks for readiness endpoints.
 
@@ -177,7 +177,7 @@ _Source:_ `docs/assets/authority/authority-rate-limit-flow.mmd`
 ## 10. Testing & Tooling
 - Unit tests: use Mongo2Go (or similar) to exercise credential stores without hitting production infrastructure (`StandardUserCredentialStoreTests` is a template).
 - Determinism: fix timestamps to UTC and sort outputs consistently; avoid random GUIDs unless stable.
-- Smoke tests: launch `dotnet run --project src/StellaOps.Authority/StellaOps.Authority` with your plug-in under `PluginBinaries/Authority` and verify `/ready`.
+- Smoke tests: launch `dotnet run --project src/StellaOps.Authority/StellaOps.Authority` with your plug-in under `StellaOps.Authority.PluginBinaries` and verify `/ready`.
 - Example verification snippet:
   ```csharp
   [Fact]
@@ -195,7 +195,7 @@ _Source:_ `docs/assets/authority/authority-rate-limit-flow.mmd`
 
 ## 11. Packaging & Delivery
 - Output assembly should follow `StellaOps.Authority.Plugin.<Name>.dll` so the host’s search pattern picks it up.
-- Place the compiled DLL plus dependencies under `PluginBinaries/Authority` for offline deployments; include hashes/signatures in release notes (Security Guild guidance forthcoming).
+- Place the compiled DLL plus dependencies under `StellaOps.Authority.PluginBinaries` for offline deployments; include hashes/signatures in release notes (Security Guild guidance forthcoming).
 - Document any external prerequisites (e.g., CA cert bundle) in your plug-in README.
 - Update `etc/authority.plugins/<plugin>.yaml` samples and include deterministic SHA256 hashes for optional bootstrap payloads when distributing Offline Kit artefacts.