Uploading a 20MB mutable fails in HTTP, but succeeds in Foolscap

[itamarst]

To reproduce: use benchmarks/upload_download.py from ​https://github.com/tahoe-lafs/tahoe-lafs/tree/3952-benchmarks, with a 20MB file. It'll run with Foolscap, but in HTTP:

>               result = await self.clients[0].create_mutable_file(MutableData(DATA))
E               allmydata.mutable.common.NotEnoughServersError: ("Publish ran out of good servers, last failure was: [Failure instance: Traceback: <class 'allmydata.storage.http_client.ClientException'>: (413, b'')\n/home/itamarst/devel/tahoe-lafs/venv/lib/python3.10/site-packages/twisted/internet/defer.py:734:errback\n/home/itamarst/devel/tahoe-lafs/venv/lib/python3.10/site-packages/twisted/internet/defer.py:797:_startRunCallbacks\n/home/itamarst/devel/tahoe-lafs/venv/lib/python3.10/site-packages/twisted/internet/defer.py:891:_runCallbacks\n/home/itamarst/devel/tahoe-lafs/venv/lib/python3.10/site-packages/twisted/internet/defer.py:1791:gotResult\n--- <exception caught here> ---\n/home/itamarst/devel/tahoe-lafs/venv/lib/python3.10/site-packages/twisted/internet/defer.py:1692:_inlineCallbacks\n/home/itamarst/devel/tahoe-lafs/venv/lib/python3.10/site-packages/twisted/python/failure.py:518:throwExceptionIntoGenerator\n/home/itamarst/devel/tahoe-lafs/src/allmydata/storage_client.py:1445:slot_testv_and_readv_and_writev\n/home/itamarst/devel/tahoe-lafs/venv/lib/python3.10/site-packages/twisted/internet/defer.py:1696:_inlineCallbacks\n/home/itamarst/devel/tahoe-lafs/src/allmydata/storage/http_client.py:868:read_test_write_chunks\n]", None)

That is, we're hitting a hard-coded size limit in the client code which presumably shouldn't be enforced given this works for Foolscap?