bug(sftp): regular OpenSSH key auth broken after asyncssh migration

[Orinks]

Description

After the asyncssh migration (#44), connecting via a regular OpenSSH private key (non-PPK) fails. The key path is set correctly but the connection does not authenticate.

Root Cause (suspected)

In protocols.py, when key_path is set, the password field is consumed as the key passphrase even if it's empty/None. asyncssh may also need client_key_passphrase set separately from password, and the two auth methods (key + password fallback) should not be mutually exclusive.

passphrase = self._info.password if self._info.password else None
connect_kwargs["client_keys"] = [key_path]
connect_kwargs["passphrase"] = passphrase  # wrong — swallows password as passphrase

Fix

• Use client_keys_passphrase (not passphrase) if the key is encrypted
• Allow password fallback alongside key auth
• Don't set passphrase=None when no passphrase is provided

[Orinks]

Dev branch handles key auth correctly via asyncssh migration. PR #51 that attempted to fix this introduced a regression and has been closed.