Address PR feedback: remove implementation details and add JSON content validation

Co-authored-by: eiriktsarpalis <2813363+eiriktsarpalis@users.noreply.github.com>

Author: Copilot

src/libraries/System.Text.Json/tests/System.Text.Json.Tests/Serialization/Value.WriteTests.cs

@@ -192,7 +192,6 @@ public static void TimeOnly_Write_Success(string value, string? expectedValue =
         public static void Int128_AsDictionaryKey_SerializesCorrectly()
         {
             // Regression test for https://github.com/dotnet/runtime/issues/116855
-            // Int128Converter.WriteAsPropertyNameCore was passing unsliced buffer to WritePropertyName
             var dict = new Dictionary<Int128, string>
             {
                 [0] = "Zero",
@@ -204,6 +203,14 @@ public static void Int128_AsDictionaryKey_SerializesCorrectly()
 
             string json = JsonSerializer.Serialize(dict);
 
+            // Validate JSON content contains expected keys and no junk data
+            Assert.Contains("\"0\"", json);
+            Assert.Contains("\"1\"", json);
+            Assert.Contains("\"-1\"", json);
+            Assert.Contains("\"170141183460469231731687303715884105727\"", json); // Int128.MaxValue
+            Assert.Contains("\"-170141183460469231731687303715884105728\"", json); // Int128.MinValue
+            Assert.DoesNotContain('\0', json); // No null characters (junk data)
+            
             // E2E validation: should roundtrip correctly without any junk data
             var deserialized = JsonSerializer.Deserialize<Dictionary<Int128, string>>(json);
             Assert.Equal(dict, deserialized);