fixed GetMeterReadingsAsync decode error

[derekwilson]

After an update to the Envoy gateway in early May 2026 it appear that the timestamp for the meter readings is now a decimal value rather than an integer.

The test app was throwing this error

 ---> System.FormatException: One of the identified items was in an invalid format.
   at NEnvoy.Internals.Converters.IntTimestampDateTimeOffsetJsonConverter.Read(Utf8JsonReader& reader, Type typeToConvert, JsonSerializerOptions options) in D:\xfer\code\NEnvoy\NEnvoy\Internals\Converters\IntTimestampDateTimeOffsetJsonConverter.cs:line 15

from this line

var meterreadings = await client.GetMeterReadingsAsync().ConfigureAwait(false);

This change just decodes the timestamp as a decimal, my understanding is that this should also work with the old int format as well.