Add tests for KeyedObjectHelper composite key methods

Introduces unit tests for encoding and decoding composite keys in the KeyedObjectHelper class, verifying correct behavior for various input scenarios.

framework/test/Volo.Abp.Core.Tests/Volo/Abp/ObjectWithKeyHelper_Tests.cs

@@ -0,0 +1,31 @@
+using System;
+using Shouldly;
+using Xunit;
+
+namespace Volo.Abp;
+
+public class KeyedObjectHelper_Tests
+{
+    [Fact]
+    public void EncodeCompositeKey()
+    {
+        var encoded = KeyedObjectHelper.EncodeCompositeKey("Book", "123");
+        encoded.ShouldBe("Qm9va3x8MTIz");
+    }
+
+    [Fact]
+    public void DecodeCompositeKey()
+    {
+        var decoded = KeyedObjectHelper.DecodeCompositeKey("Qm9va3x8MTIz");
+        decoded.ShouldBe("Book||123");
+    }
+
+    [Fact]
+    public void Encode_Decode_CompositeKey()
+    {
+        var encoded = KeyedObjectHelper.EncodeCompositeKey("User", 42, Guid.Empty);
+        var decoded = KeyedObjectHelper.DecodeCompositeKey(encoded);
+        
+        decoded.ShouldBe($"User||42||{Guid.Empty}");
+    }
+}