Fix case-insensitive JSON deserialization of default enum vals

[PranavSenthilnathan]

Fixes #110745

The JSON serializer supports deserializing an enum from the default name (the C# enum member name) even when a naming policy is used. For example, both "enum_value" and "EnumValue" will be deserialized to MyEnum.EnumValue when lower snake casing is specified:

enum MyEnum { EnumValue }

However, there is a regression in .NET 9.0 when the enum default name is the same as the name from the naming policy:

// Both work in .NET 8.0 but throw in .NET 9.0
Console.WriteLine(JsonSerializer.Deserialize<MyEnum>("\"Example\"", options));
Console.WriteLine(JsonSerializer.Deserialize<MyEnum>("\"Another_Example\"", options));

enum MyEnum
{ 
  example,
  another_example
}

This PR changes this back to the .NET 8.0 behavior.

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