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/10_PLUGIN_SDK_GUIDE.md

@@ -82,28 +82,53 @@ Add this to **`MyPlugin.Schedule.csproj`** so the signed DLL + `.sig` land in th
 
 ---
 
-## 5 Dependency‑Injection Entry‑point  
-
-Back‑end auto‑discovers the static method below:
-
-~~~csharp
-namespace StellaOps.DependencyInjection;
-
-public static class IoCConfigurator
-{
-    public static IServiceCollection Configure(this IServiceCollection services,
-                                               IConfiguration cfg)
-    {
-        services.AddSingleton<IJob, MyJob>();        // schedule job
-        services.Configure<MyPluginOptions>(cfg.GetSection("Plugins:MyPlugin"));
-        return services;
-    }
-}
-~~~
-
----
-
-## 6 Schedule Plug‑ins  
+## 5 Dependency‑Injection Entry‑point  
+
+Back‑end auto‑discovers restart‑time bindings through two mechanisms:
+
+1. **Service binding metadata** for simple contracts.
+2. **`IDependencyInjectionRoutine`** implementations when you need full control.
+
+### 5.1 Service binding metadata
+
+Annotate implementations with `[ServiceBinding]` to declare their lifetime and service contract.  
+The loader honours scoped lifetimes and will register the service before executing any custom DI routines.
+
+~~~csharp
+using Microsoft.Extensions.DependencyInjection;
+using StellaOps.DependencyInjection;
+
+[ServiceBinding(typeof(IJob), ServiceLifetime.Scoped, RegisterAsSelf = true)]
+public sealed class MyJob : IJob
+{
+    // IJob dependencies can now use scoped services (Mongo sessions, etc.)
+}
+~~~
+
+Use `RegisterAsSelf = true` when you also want to resolve the concrete type.  
+Set `ReplaceExisting = true` to override default descriptors if the host already provides one.
+
+### 5.2 Dependency injection routines
+
+For advanced scenarios continue to expose a routine:
+
+~~~csharp
+namespace StellaOps.DependencyInjection;
+
+public sealed class IoCConfigurator : IDependencyInjectionRoutine
+{
+    public IServiceCollection Register(IServiceCollection services, IConfiguration cfg)
+    {
+        services.AddSingleton<IJob, MyJob>();        // schedule job
+        services.Configure<MyPluginOptions>(cfg.GetSection("Plugins:MyPlugin"));
+        return services;
+    }
+}
+~~~
+
+---
+
+## 6 Schedule Plug‑ins  
 
 ### 6.1 Minimal Job  
 
@@ -191,4 +216,4 @@ On merge, the plug‑in shows up in the UI Marketplace.
 | NotDetected         | .sig missing               | cosign sign …                               |
 | VersionGateMismatch | Backend 2.1 vs plug‑in 2.0 | Re‑compile / bump attribute                 |
 | FileLoadException   | Duplicate                  | StellaOps.Common	Ensure PrivateAssets="all" |
-| Redis               | timeouts	Large writes      | Batch or use Mongo                          |
+| Redis               | timeouts	Large writes      | Batch or use Mongo                          |