Investigate whether GetRawText should be used instead of ToString for generated deserialization code

[JoshLove-msft]

For code paths where we are calling into JsonSerializer.Deserialize, we would need to use GetRawText if the underlying JSON type can be something other than object. This has come up in #2852.

In that PR, I've updated the specific case that DFE and SystemData falls under, but there are several other places in JsonCodeWriterExtensions where we pass ToString to JsonSerializer.Deserialize that should be evaluated for the same change.