Adds testSmtpConnection() that performs EHLO + auth via JavaMailSender
before persisting to Logto — saves fail fast with a clear error if
SMTP credentials are wrong. Password is now optional when editing:
if left blank the backend fetches the existing password from Logto's
connector config, so users can update host/port/fromEmail without
re-entering the password every time.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>