11898.Bugfix

Restored :py:meth:~aiohttp.BodyPartReader.decode as a synchronous method for backward compatibility. The method was inadvertently changed to async in 3.13.3 as part of the decompression bomb security fix. A new :py:meth:~aiohttp.BodyPartReader.decode_iter method is now available for non-blocking decompression of large payloads using an async generator. Internal aiohttp code uses the async variant to maintain security protections.

Changed multipart processing chunk sizes from 64 KiB to 256KiB, to better match aiohttp internals -- by :user:bdraco and :user:Dreamsorcerer.