Fix data loss when bucketized payload is an exact multiple of bucket size

[LZD-PratyushBhatt]

The bucketized writer sliced the final bucket using "compressedRecord.length % _bucketSize". When the GZIP compressed payload length happens to be an exact multiple of the bucket size that remainder is zero, so the last bucket was written empty and its bytes were dropped. The reader then reassembled a truncated buffer and every subsequent read of that path failed with "Failed to decompress path". The record was effectively unrecoverable until the next write landed on a non-multiple length.

The reader had the mirror image of the same bug in its final-bucket arraycopy, and it also advanced its copy pointer only on the non-final branch, so the final chunk was copied to the wrong offset.

Slice and copy to the end of the payload instead of relying on the remainder, which is correct for both the exact-multiple and the partial final bucket cases.

The reader additionally computed the bucket count from its own configured _bucketSize rather than the BUCKET_SIZE the writer recorded in the metadata ZNode. An accessor constructed with a different bucket size than the writer therefore looked for the wrong number of buckets and failed the same way. Use the persisted bucket size, which is the authoritative value, and fail with an explicit message if a bucket is missing or short rather than throwing an opaque ArrayIndexOutOfBoundsException.

Both cases are covered by new tests that fail without this fix.

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