Bug-class coverage for both fixes: the full catalogue survives Save, context
lengths are preserved, the key never lands in config.yaml on either write
path, blank clears it, a pre-fix plaintext key migrates while a ${VAR}
template is left alone, two endpoints on one host keep separate credentials,
and an IP-derived name is still a valid POSIX env var.
The two delete tests asserted on the plaintext mirror; they now assert the
same invariants against the credential reference.
