MAX_JSON was 1 GiB on every target, but on wasm32 the 4 GiB address space
also holds the document the JSON deserializes into, several times its size.
The cap is now a quarter of the native one there, mirroring MAX_BLOBS,
and Unpacked::within makes the cap exercisable wherever a test runs.
Co-authored-by: rahul05ranjan <11793455+rahul05ranjan@users.noreply.github.com>