feat: update terminal support messaging and enhance guest agent provisioning logic

uvm/guest_exec.py

@@ -71,8 +71,8 @@ class GuestSession:
             if connection is not None:
                 connection.close()
             raise GuestAgentUnavailableError(
-                "guest agent is unavailable or not ready; install it with POST /vms/{vm_id}/agent/install "
-                "while stopped, then start the VM (vsock-capable kernel required)"
+                "guest agent is unavailable or not ready; wait for guest startup, or restart the VM "
+                "to apply automatic vsock guest-agent setup (vsock-capable kernel required)"
             ) from error
 
     def execute(self, command: str, *, sudo: bool, timeout_seconds: float) -> ExecResult: