The codec streaming GET reader divides by erasure.block_size in
build_codec_streaming_part_reader without validating the erasure
dimensions, unlike the legacy multipart path which already rejects
block_size==0 / data_shards==0. FileInfo::is_valid() does not check
block_size, so corrupted on-disk metadata (block_size==0, data_blocks>0)
passes validation and panics the read task with a divide-by-zero.
Add Erasure::has_valid_dimensions() and reject invalid dimensions at the
codec streaming entry before any disk access, mirroring the legacy guard
(which now reuses the same predicate).
Refs backlog#868 (868-1).
