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.

src/StellaOps.Notify.Engine/ChannelTestPreviewContracts.cs

@@ -0,0 +1,84 @@
+using System;
+using System.Collections.Generic;
+using System.Security.Cryptography;
+using System.Text;
+using System.Threading;
+using System.Threading.Tasks;
+using StellaOps.Notify.Models;
+
+namespace StellaOps.Notify.Engine;
+
+/// <summary>
+/// Contract implemented by Notify channel plug-ins to generate channel-specific test preview payloads.
+/// </summary>
+public interface INotifyChannelTestProvider
+{
+    /// <summary>
+    /// Channel type supported by the provider.
+    /// </summary>
+    NotifyChannelType ChannelType { get; }
+
+    /// <summary>
+    /// Builds a channel-specific preview for a test-send request.
+    /// </summary>
+    Task<ChannelTestPreviewResult> BuildPreviewAsync(ChannelTestPreviewContext context, CancellationToken cancellationToken);
+}
+
+/// <summary>
+/// Sanitised request payload passed to channel plug-ins when building a preview.
+/// </summary>
+public sealed record ChannelTestPreviewRequest(
+    string? TargetOverride,
+    string? TemplateId,
+    string? Title,
+    string? Summary,
+    string? Body,
+    string? TextBody,
+    string? Locale,
+    IReadOnlyDictionary<string, string> Metadata,
+    IReadOnlyList<string> Attachments);
+
+/// <summary>
+/// Immutable context describing the channel and request for a test preview.
+/// </summary>
+public sealed record ChannelTestPreviewContext(
+    string TenantId,
+    NotifyChannel Channel,
+    string Target,
+    ChannelTestPreviewRequest Request,
+    DateTimeOffset Timestamp,
+    string TraceId);
+
+/// <summary>
+/// Result returned by channel plug-ins for test preview generation.
+/// </summary>
+public sealed record ChannelTestPreviewResult(
+    NotifyDeliveryRendered Preview,
+    IReadOnlyDictionary<string, string>? Metadata);
+
+/// <summary>
+/// Exception thrown by plug-ins when preview input is invalid.
+/// </summary>
+public sealed class ChannelTestPreviewException : Exception
+{
+    public ChannelTestPreviewException(string message)
+        : base(message)
+    {
+    }
+}
+
+/// <summary>
+/// Shared helpers for channel preview generation.
+/// </summary>
+public static class ChannelTestPreviewUtilities
+{
+    /// <summary>
+    /// Computes a lowercase hex SHA-256 body hash for preview payloads.
+    /// </summary>
+    public static string ComputeBodyHash(string body)
+    {
+        var bytes = Encoding.UTF8.GetBytes(body);
+        var hash = SHA256.HashData(bytes);
+        return Convert.ToHexString(hash).ToLowerInvariant();
+    }
+}

src/StellaOps.Notify.Engine/StellaOps.Notify.Engine.csproj

@@ -4,4 +4,8 @@
     <ImplicitUsings>enable</ImplicitUsings>
     <Nullable>enable</Nullable>
   </PropertyGroup>
+
+  <ItemGroup>
+    <ProjectReference Include="..\StellaOps.Notify.Models\StellaOps.Notify.Models.csproj" />
+  </ItemGroup>
 </Project>