fix: address PR review feedback for AOT serializer cleanup

- Add TrySetDefault to EventSerializer using Interlocked.CompareExchange
  so DefaultEventSerializer constructor doesn't overwrite an existing
  serializer configuration
- Add module initializer to Eventuous.Tests.Subscriptions.Base so
  integration tests (Azure, Kafka, KurrentDB) have EventSerializer.Default
  configured before use

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: alexeyzimarev

src/Core/src/Eventuous.Serialization.Json/DefaultEventSerializer.cs

@@ -16,7 +16,7 @@ public DefaultEventSerializer(JsonSerializerOptions options, ITypeMapper? typeMa
         _typeMapper = typeMapper ?? TypeMap.Instance;
 
         // Auto-register as default if none is set
-        EventSerializer.SetDefault(this);
+        EventSerializer.TrySetDefault(this);
     }
 
     [UnconditionalSuppressMessage("Trimming", "IL2026", Justification = "This class is not intended for AOT use.")]

src/Core/src/Eventuous.Serialization/EventSerializer.cs

@@ -26,4 +26,14 @@ public static class EventSerializer {
     /// </summary>
     public static void SetDefault(IEventSerializer serializer)
         => _default = serializer ?? throw new ArgumentNullException(nameof(serializer));
+
+    /// <summary>
+    /// Sets the default event serializer only if one has not already been configured.
+    /// Returns true if the default was set, false if it was already configured.
+    /// </summary>
+    public static bool TrySetDefault(IEventSerializer serializer) {
+        ArgumentNullException.ThrowIfNull(serializer);
+
+        return Interlocked.CompareExchange(ref _default, serializer, null) == null;
+    }
 }

src/Core/test/Eventuous.Tests.Subscriptions.Base/Eventuous.Tests.Subscriptions.Base.csproj

@@ -6,6 +6,7 @@
     </PropertyGroup>
     <ItemGroup>
         <ProjectReference Include="$(LocalRoot)\Eventuous.Subscriptions\Eventuous.Subscriptions.csproj" />
+        <ProjectReference Include="$(CoreRoot)\Eventuous.Serialization.Json\Eventuous.Serialization.Json.csproj"/>
         <ProjectReference Include="..\Eventuous.Tests.Persistence.Base\Eventuous.Tests.Persistence.Base.csproj"/>
         <ProjectReference Include="$(ExtRoot)\Eventuous.Extensions.DependencyInjection\Eventuous.Extensions.DependencyInjection.csproj"/>
     </ItemGroup>

src/Core/test/Eventuous.Tests.Subscriptions.Base/TestSetup.cs

@@ -0,0 +1,10 @@
+using System.Runtime.CompilerServices;
+using System.Text.Json;
+
+namespace Eventuous.Tests.Subscriptions.Base;
+
+static class TestSetup {
+    [ModuleInitializer]
+    internal static void Initialize()
+        => new DefaultEventSerializer(new JsonSerializerOptions(JsonSerializerDefaults.Web));
+}