Canonicalize uploaded OpenSSH private keys before validation by
normalizing line endings, removing whitespace from the base64 body,
and re-wrapping it between the BEGIN and END markers.
Add SSH credential tests that generate a real ed25519 key, damage its
wrapping, and verify that validation succeeds after normalization.
Return a clearer validation error for PEM private keys, which are not
supported by the current credential flow.
