Fix/Adopt New DateTime XML Spector Scenario

The new [ModelWithDatetime](https://github.com/microsoft/typespec/blob/main/packages/http-specs/specs/payload/xml/mockapi.ts#L132) xml scenario is failing since the spector tests expects the `rfc3339` property in the request payload to only have 3 digits of precision. For rfc3339, we default to 7 digits.

We should validate what the expected behavior should be for .NET specifically, and/or if the spector test itself needs to be relaxed.