refactor: OnMessageReceived in ein Ereignis umgewandelt für bessere Ereignisbehandlung

- `OnMessageReceived` von einem Delegaten in ein Ereignis umgewandelt, um die Ereignisabonnierung und -behandlung zu verbessern.

DigitalData.Auth.Client/ClientParams.cs

@@ -25,7 +25,7 @@ public class ClientParams
     /// </summary>
     public TimeSpan? RetryDelay { get; set; }
 
-    public ClientEvent OnMessageReceived = delegate { };
+    public event ClientEvent OnMessageReceived = delegate { };
 
     internal void TriggerOnMessageReceived(AuthClient client, string issuer, string audience, string key, ILogger? logger = null)
         => OnMessageReceived(client, issuer, audience, key, logger);

DigitalData.Auth.Tests/API/AuthHubTests.cs

@@ -98,7 +98,7 @@ public class AuthHubTests
         var provider_receiver = Build(opt =>
         {
             opt.Url = _hubUrl;
-            opt.Events.OnMessageReceived = (issuer, audience, key, logger) =>
+            opt.OnMessageReceived += (client, issuer, audience, key, logger) =>
             {
                 rcv_issuer = issuer;
                 rcv_audience = audience;

DigitalData.Auth.Tests/Client/AuthClientTests.cs

@@ -129,7 +129,7 @@ public class AuthClientTests
         var provider_receiver = Build(opt =>
         {
             opt.Url = _hubUrl;
-            opt.Events.OnMessageReceived = (issuer, audience, key, logger) =>
+            opt.OnMessageReceived += (client, issuer, audience, key, logger) =>
             {
                 rcv_issuer = issuer;
                 rcv_audience = audience;
@@ -166,7 +166,7 @@ public class AuthClientTests
         var provider = Build(opt =>
         {
             opt.Url = _hubUrl;
-            opt.Events.OnMessageReceived = (issuer, audience, key, logger) => publicKey = key;
+            opt.OnMessageReceived += (client, issuer, audience, key, logger) => publicKey = key;
         });
         var client = provider.GetRequiredService<IAuthClient>();
         await client.StartAsync();