Fix buffer slicing in (U)Int128 property name serialization (#116868)

* Initial plan for issue

* Fix buffer slicing in (U)Int128 property name serialization

Co-authored-by: PranavSenthilnathan <12225508+PranavSenthilnathan@users.noreply.github.com>

* Add regression tests for Int128/UInt128 property name serialization

Co-authored-by: PranavSenthilnathan <12225508+PranavSenthilnathan@users.noreply.github.com>

* Address PR feedback: simplify test assertions and remove junk data checks

Co-authored-by: PranavSenthilnathan <12225508+PranavSenthilnathan@users.noreply.github.com>

* Add JSON content validation to Int128/UInt128 dictionary tests

Co-authored-by: PranavSenthilnathan <12225508+PranavSenthilnathan@users.noreply.github.com>

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

* Replace multiple assertions with single E2E validation in Int128 dictionary test

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

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

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

* Replace Assert.Contains with Assert.Equal for E2E JSON validation in Int128/UInt128 tests

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

* Restore multi-key dictionaries in Int128/UInt128 tests as requested

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

* Use raw string literals for JSON assertions in Int128/UInt128 tests

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

* Remove superficial comments from Int128/UInt128 dictionary tests

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

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: PranavSenthilnathan <12225508+PranavSenthilnathan@users.noreply.github.com>
Co-authored-by: Eirik Tsarpalis <eirik.tsarpalis@gmail.com>
Co-authored-by: eiriktsarpalis <2813363+eiriktsarpalis@users.noreply.github.com>

Author: 3 people

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Value/Int128Converter.cs

@@ -73,7 +73,7 @@ internal override void WriteAsPropertyNameCore(Utf8JsonWriter writer, Int128 val
         {
             Span<byte> buffer = stackalloc byte[MaxFormatLength];
             Format(buffer, value, out int written);
-            writer.WritePropertyName(buffer);
+            writer.WritePropertyName(buffer.Slice(0, written));
         }
 
         internal override Int128 ReadNumberWithCustomHandling(ref Utf8JsonReader reader, JsonNumberHandling handling, JsonSerializerOptions options)

src/libraries/System.Text.Json/src/System/Text/Json/Serialization/Converters/Value/UInt128Converter.cs

@@ -73,7 +73,7 @@ internal override void WriteAsPropertyNameCore(Utf8JsonWriter writer, UInt128 va
         {
             Span<byte> buffer = stackalloc byte[MaxFormatLength];
             Format(buffer, value, out int written);
-            writer.WritePropertyName(buffer);
+            writer.WritePropertyName(buffer.Slice(0, written));
         }
 
         internal override UInt128 ReadNumberWithCustomHandling(ref Utf8JsonReader reader, JsonNumberHandling handling, JsonSerializerOptions options)

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

@@ -187,6 +187,44 @@ public static void TimeOnly_Write_Success(string value, string? expectedValue =
             string json = JsonSerializer.Serialize(ts);
             Assert.Equal($"\"{expectedValue ?? value}\"", json);
         }
+
+        [Fact]
+        public static void Int128_AsDictionaryKey_SerializesCorrectly()
+        {
+            // Regression test for https://github.com/dotnet/runtime/issues/116855
+            var dict = new Dictionary<Int128, string>
+            {
+                [0] = "Zero",
+                [1] = "One",
+                [-1] = "MinusOne",
+                [Int128.MaxValue] = "Max",
+                [Int128.MinValue] = "Min"
+            };
+
+            string json = JsonSerializer.Serialize(dict);
+            Assert.Equal("""{"0":"Zero","1":"One","-1":"MinusOne","170141183460469231731687303715884105727":"Max","-170141183460469231731687303715884105728":"Min"}""", json);
+            
+            var deserialized = JsonSerializer.Deserialize<Dictionary<Int128, string>>(json);
+            Assert.Equal(dict, deserialized);
+        }
+
+        [Fact]
+        public static void UInt128_AsDictionaryKey_SerializesCorrectly()
+        {
+            // Regression test for https://github.com/dotnet/runtime/issues/116855
+            var dict = new Dictionary<UInt128, string>
+            {
+                [0] = "Zero", 
+                [1] = "One",
+                [UInt128.MaxValue] = "Max"
+            };
+
+            string json = JsonSerializer.Serialize(dict);
+            Assert.Equal("""{"0":"Zero","1":"One","340282366920938463463374607431768211455":"Max"}""", json);
+            
+            var deserialized = JsonSerializer.Deserialize<Dictionary<UInt128, string>>(json);
+            Assert.Equal(dict, deserialized);
+        }
 #endif
     }
 }