JsonConverter.Read/WriteAsPropertyName methods throw NotSupportedException when invoked in built-in converters #77326
Description
The code
var options = new JsonSerializerOptions();
var writer = new Utf8JsonWriter(new MemoryStream());
writer.WriteStartObject();
JsonConverter<int> converter = JsonMetadataServices.Int32Converter;
converter.WriteAsPropertyName(writer, 42, options);throws the following exception
Unhandled exception. System.NotSupportedException: The type 'System.Int32' is not a supported dictionary key using converter of type 'System.Text.Json.Serialization.Converters.Int32Converter'.
at System.Text.Json.ThrowHelper.ThrowNotSupportedException_DictionaryKeyTypeNotSupported(Type keyType, JsonConverter converter)
at System.Text.Json.Serialization.JsonConverter`1.WriteAsPropertyName(Utf8JsonWriter writer, T value, JsonSerializerOptions options)
at Program.<Main>$(String[] args)
Even though the converter does support serializing to dictionary keys.
The issue does not manifest when the serializer calls into the built-in converters because it follows uses the Read/WriteAsPropertyNameCore methods instead. For consistency, we should make sure that the public methods behave as expected when invoked by user code.