"not a valid OPENSSH private key file" — building a compat layer for seven SSH private-key formats

If you wire SSH into WordPress maintenance automation, you'll meet this error sooner or later:

SSHException: not a valid OPENSSH private key file

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"I configured the key file — why?" is the usual reaction. Tracing several real SSH-connection-test failures, the root issue becomes clear: paramiko alone can't read SSH private keys outside the OpenSSH format.