UInt64.CreateSaturating<Int128> truncates instead of saturates (#96369)

* UInt64.CreateSaturating<Int128> truncates instead of saturates

* Update src/libraries/System.Private.CoreLib/src/System/Int128.cs

---------

Co-authored-by: Stephen Toub <stoub@microsoft.com>
Co-authored-by: Tanner Gooding <tagoo@outlook.com>

Author: 3 people

src/libraries/System.Private.CoreLib/src/System/Int128.cs

@@ -1904,7 +1904,8 @@ static bool INumberBase<Int128>.TryConvertToSaturating<TOther>(Int128 value, [Ma
             }
             else if (typeof(TOther) == typeof(ulong))
             {
-                ulong actualResult = (value <= 0) ? ulong.MinValue : (ulong)value;
+                ulong actualResult = (value >= ulong.MaxValue) ? ulong.MaxValue :
+                                     (value <= ulong.MinValue) ? ulong.MinValue : (ulong)value;
                 result = (TOther)(object)actualResult;
                 return true;
             }

src/libraries/System.Runtime/tests/System.Runtime.Tests/System/UInt64Tests.GenericMath.cs

@@ -1646,6 +1646,7 @@ public static void CreateSaturatingFromInt128Test()
             Assert.Equal((ulong)0xFFFF_FFFF_FFFF_FFFF, NumberBaseHelper<ulong>.CreateSaturating<Int128>(Int128.MaxValue));
             Assert.Equal((ulong)0x0000_0000_0000_0000, NumberBaseHelper<ulong>.CreateSaturating<Int128>(Int128.MinValue));
             Assert.Equal((ulong)0x0000_0000_0000_0000, NumberBaseHelper<ulong>.CreateSaturating<Int128>(Int128.NegativeOne));
+            Assert.Equal((ulong)0xFFFF_FFFF_FFFF_FFFF, NumberBaseHelper<ulong>.CreateSaturating<Int128>((Int128)ulong.MaxValue + (Int128)10L));
         }
 
         [Fact]