Investigate situations where JsonDecodeException might give a wrong result for path

[bowbahdoe]

Technically speaking, we could end up with an incorrect "path" recorded in a JsonDecodeException if someone escapes the decoder mechanism.

For instance.

Json j = Json.readString("""
    {
      "a": [
         {
            "b": 123
         }
      ]
    }
""");

List<String> b = JsonDecoder.field(j, "a", a -> {
    var array = JsonDecoder.array(a);
    return array.stream()
        .map(JsonDecoder::string)
        .toList();
} 

This would record the path as a.b not a.[0].b. There might not be a nice way to account for this in general, at least without propagating metadata to the actual Json forms.

Maybe metadata is valid to propagate though! There could be more uses.

[bowbahdoe]

Idea that popped in my head this night - If extent locals really are cheap enough, we could mark "trusted contexts" where we know that all accesses have a matching throw atIndex/atField and know that those add to the path, representing "untrusted contexts" in a different way that isn't "this error definitely occurred at this path".

Bit unsure of the shape of that atm, but its one idea