Stop the AWS driver claiming server_checksums it does not provide

Stop the AWS driver claiming server_checksums it does not provide aws() declared server_checksums=true, but put sets no checksum_algorithm and head no checksum_mode, so on real S3 the checksum is always None and the integrity scrub's checksum cross-check (which only fires when both sides have one) silently never ran. Declare false so the capability is truthful; the scrub relies on size + a first-byte probe. Earning it fully (put/multipart/copy + head checksum_mode) is tracked separately.