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.Authority/StellaOps.Authority.Plugin.Standard.Tests/StandardClientProvisioningStoreTests.cs

@@ -1,4 +1,6 @@
+using System;
 using System.Collections.Generic;
+using System.Linq;
 using System.Threading;
 using System.Threading.Tasks;
 using MongoDB.Driver;
@@ -43,6 +45,74 @@ public class StandardClientProvisioningStoreTests
         Assert.Contains("scopeA", descriptor.AllowedScopes);
     }
 
+    [Fact]
+    public async Task CreateOrUpdateAsync_StoresAudiences()
+    {
+        var store = new TrackingClientStore();
+        var revocations = new TrackingRevocationStore();
+        var provisioning = new StandardClientProvisioningStore("standard", store, revocations, TimeProvider.System);
+
+        var registration = new AuthorityClientRegistration(
+            clientId: "signer",
+            confidential: false,
+            displayName: "Signer",
+            clientSecret: null,
+            allowedGrantTypes: new[] { "client_credentials" },
+            allowedScopes: new[] { "signer.sign" },
+            allowedAudiences: new[] { "attestor", "signer" });
+
+        var result = await provisioning.CreateOrUpdateAsync(registration, CancellationToken.None);
+
+        Assert.True(result.Succeeded);
+        var document = Assert.Contains("signer", store.Documents);
+        Assert.Equal("attestor signer", document.Value.Properties[AuthorityClientMetadataKeys.Audiences]);
+
+        var descriptor = await provisioning.FindByClientIdAsync("signer", CancellationToken.None);
+        Assert.NotNull(descriptor);
+        Assert.Equal(new[] { "attestor", "signer" }, descriptor!.Audiences.OrderBy(value => value, StringComparer.Ordinal));
+    }
+
+    [Fact]
+    public async Task CreateOrUpdateAsync_MapsCertificateBindings()
+    {
+        var store = new TrackingClientStore();
+        var revocations = new TrackingRevocationStore();
+        var provisioning = new StandardClientProvisioningStore("standard", store, revocations, TimeProvider.System);
+
+        var bindingRegistration = new AuthorityClientCertificateBindingRegistration(
+            thumbprint: "aa:bb:cc:dd",
+            serialNumber: "01ff",
+            subject: "CN=mtls-client",
+            issuer: "CN=test-ca",
+            subjectAlternativeNames: new[] { "client.mtls.test", "spiffe://client" },
+            notBefore: DateTimeOffset.UtcNow.AddMinutes(-5),
+            notAfter: DateTimeOffset.UtcNow.AddHours(1),
+            label: "primary");
+
+        var registration = new AuthorityClientRegistration(
+            clientId: "mtls-client",
+            confidential: true,
+            displayName: "MTLS Client",
+            clientSecret: "secret",
+            allowedGrantTypes: new[] { "client_credentials" },
+            allowedScopes: new[] { "signer.sign" },
+            allowedAudiences: new[] { "signer" },
+            certificateBindings: new[] { bindingRegistration });
+
+        await provisioning.CreateOrUpdateAsync(registration, CancellationToken.None);
+
+        var document = Assert.Contains("mtls-client", store.Documents).Value;
+        var binding = Assert.Single(document.CertificateBindings);
+        Assert.Equal("AABBCCDD", binding.Thumbprint);
+        Assert.Equal("01ff", binding.SerialNumber);
+        Assert.Equal("CN=mtls-client", binding.Subject);
+        Assert.Equal("CN=test-ca", binding.Issuer);
+        Assert.Equal(new[] { "client.mtls.test", "spiffe://client" }, binding.SubjectAlternativeNames);
+        Assert.Equal(bindingRegistration.NotBefore, binding.NotBefore);
+        Assert.Equal(bindingRegistration.NotAfter, binding.NotAfter);
+        Assert.Equal("primary", binding.Label);
+    }
+
     private sealed class TrackingClientStore : IAuthorityClientStore
     {
         public Dictionary<string, AuthorityClientDocument> Documents { get; } = new(StringComparer.OrdinalIgnoreCase);