Incompatible with JsonTreeWriter

[ZacSweers]

When the incoming GSON JsonWriter is JsonTreeWriter, jsonValue() APIs will no longer work because GSON doesn't support writing raw JSON in this environment. Not sure if this is something we really can support, vs not allowing Moshi serializers to be preferred when we're in this condition. Not sure we can infer this ahead of time though.

[ZacSweers]

One idea could be to propose adding support for this in GSON via internal holder type, so that when it tries to convert it to string it just takes the raw byte out of this holder

[ZacSweers]

I found these but nothing promising

google/gson#1268
google/gson#1289
google/gson#1651

Put up a PR here google/gson#1819.

GSON seems mostly abandoned though so not optimistic about it getting reviewed/merged. Those that need this should probably either move away from using JsonElement APIs in their own custom adapters or creating a fork with the above change included.

I think it would be possible to replace the built-in JsonTreeWriter in a custom version (i.e. the one from my PR) + TypeAdapters.JSON_ELEMENT replacement with the PR changes, but hesitant to do that unless we have to though. I tried to implement it without upstream support, but hit a dead end due to Streams.java in gson eventually using its own hardcoded TypeAdapters.JSON_ELEMENT to write it in the end.