test: finalize CI test coverage and fix dependency import path

proxy_server.py

@@ -344,7 +344,7 @@ class SMTPProxyHandler:
         await asyncio.to_thread(self.send_message, envelope.mail_from, envelope.rcpt_tos, envelope.content)
         return "250 Message accepted for delivery"
 
-        def send_message(self, sender, recipients, data):
+    def send_message(self, sender, recipients, data):
         """Forward a complete SMTP message to the backend SMTP server."""
         message = data.decode("utf-8", errors="replace")
         if Settings.BACKEND_SMTP_USE_SSL: