Added unit testing

Syrette.Tests/ServiceContainerThreadingTests.cs

@@ -0,0 +1,61 @@
+namespace Syrette.Tests;
+
+public class ServiceContainerThreadingTests
+{
+    [Fact]
+    public void Concurrent_singleton_resolution_returns_same_instance()
+    {
+        var container = new ServiceContainer();
+        container.AddSingleton<ITestService, TestServiceImpl>();
+
+        var results = new ITestService[10];
+
+        Parallel.For(0, 10, i =>
+        {
+            results[i] = container.GetService<ITestService>();
+        });
+
+        for (int i = 1; i < results.Length; i++)
+        {
+            Assert.Same(results[0], results[i]);
+        }
+    }
+
+    [Fact]
+    public void Concurrent_transient_resolution_returns_unique_instances()
+    {
+        var container = new ServiceContainer();
+        container.AddTransient<ITestService, TestServiceImpl>();
+
+        var results = new ITestService[10];
+
+        Parallel.For(0, 10, i =>
+        {
+            results[i] = container.GetService<ITestService>();
+        });
+
+        for (int i = 0; i < results.Length; i++)
+        {
+            for (int j = i + 1; j < results.Length; j++)
+            {
+                Assert.NotSame(results[i], results[j]);
+            }
+        }
+    }
+
+    [Fact]
+    public void Concurrent_registration_and_resolution_does_not_crash()
+    {
+        var container = new ServiceContainer();
+
+        Parallel.Invoke(
+            () => { try { container.AddSingleton<ITestService, TestServiceImpl>(); } catch { } },
+            () => { try { container.AddSingleton<ITestService, TestServiceImpl>(); } catch { } },
+            () => { try { container.GetService<ITestService>(); } catch { } },
+            () => { try { container.AddSingleton<ITestService, TestServiceImpl>(); } catch { } },
+            () => { try { container.GetService<ITestService>(); } catch { } }
+        );
+
+        Assert.NotNull(container);
+    }
+}