aLongKeyIsStillAKey

A key considerably longer than the hand-written names a caller normally uses.

The knob is LONG_KEY_CHARS, and it is bounded from above on purpose. A POSIX filesystem refuses a single path component past 255 bytes outright, so a backend mapping a key to a filename cannot satisfy an arbitrarily long key without hashing — and the contract never promised one. 128 characters is comfortably past every key any in-tree caller uses and comfortably inside that limit, so what this property checks is that nothing truncates a long key (which would fold it together with any sibling sharing its first N characters — the distinctKeysAddressDistinctEntries failure arriving by a different route).

Said plainly: this does not establish that a 4 KiB key works, and a backend should not be read as promising one. The two keys differ only in their last character, so a truncating backend fails rather than passing quietly.