fix(sftp): fix OpenSSH key auth and password fallback after asyncssh migration

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Summary

Fixes #49 — regular OpenSSH key authentication broken after asyncssh migration.

Three bugs in SFTPClient.connect():

• Password used as key passphrase unconditionally: the password field was always passed as passphrase to asyncssh.connect(), even for non-encrypted keys. Removed — passphrase is no longer set at all, letting asyncssh handle encrypted keys via its own KeyImportError path (already caught).
• passphrase=None explicitly set: even when no password existed, passphrase=None was passed in kwargs. Now omitted entirely per asyncssh best practice.
• No password fallback with key auth: elif prevented password from being passed alongside client_keys. Changed to separate if blocks so password auth works as fallback when key auth fails.

Also added a distinct PermissionDenied error message for the key+password case.

Test plan

• test_connect_with_key_file — key-only: no passphrase or password in kwargs
• test_connect_with_key_and_password_fallback — key+password: password kwarg set, no passphrase
• test_connect_with_key_and_password_logs_both_methods — auth methods log includes both
• test_auth_failure_message_for_key_and_password — error message mentions password fallback
• All 93 existing protocol tests pass

🤖 Generated with Claude Code

[Orinks]

Closing — this PR introduces a regression where key-only auth (no password) stops working. Dev branch already handles key auth correctly. Issue #49 may be invalid.