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.Concelier.WebService/Services/MirrorRateLimiter.cs

@@ -0,0 +1,57 @@
+using Microsoft.Extensions.Caching.Memory;
+
+namespace StellaOps.Concelier.WebService.Services;
+
+internal sealed class MirrorRateLimiter
+{
+    private readonly IMemoryCache _cache;
+    private readonly TimeProvider _timeProvider;
+    private static readonly TimeSpan Window = TimeSpan.FromHours(1);
+
+    public MirrorRateLimiter(IMemoryCache cache, TimeProvider timeProvider)
+    {
+        _cache = cache ?? throw new ArgumentNullException(nameof(cache));
+        _timeProvider = timeProvider ?? throw new ArgumentNullException(nameof(timeProvider));
+    }
+
+    public bool TryAcquire(string domainId, string scope, int limit, out TimeSpan? retryAfter)
+    {
+        retryAfter = null;
+
+        if (limit <= 0 || limit == int.MaxValue)
+        {
+            return true;
+        }
+
+        var key = CreateKey(domainId, scope);
+        var now = _timeProvider.GetUtcNow();
+
+        var counter = _cache.Get<Counter>(key);
+        if (counter is null || now - counter.WindowStart >= Window)
+        {
+            counter = new Counter(now, 0);
+        }
+
+        if (counter.Count >= limit)
+        {
+            var windowEnd = counter.WindowStart + Window;
+            retryAfter = windowEnd > now ? windowEnd - now : TimeSpan.Zero;
+            return false;
+        }
+
+        counter = counter with { Count = counter.Count + 1 };
+        var absoluteExpiration = counter.WindowStart + Window;
+        _cache.Set(key, counter, absoluteExpiration);
+        return true;
+    }
+
+    private static string CreateKey(string domainId, string scope)
+        => string.Create(domainId.Length + scope.Length + 1, (domainId, scope), static (span, state) =>
+        {
+            state.domainId.AsSpan().CopyTo(span);
+            span[state.domainId.Length] = '|';
+            state.scope.AsSpan().CopyTo(span[(state.domainId.Length + 1)..]);
+        });
+
+    private sealed record Counter(DateTimeOffset WindowStart, int Count);
+}