Handle missing MFA method in login process

[Amodio]

Fix #72

Raise error if no valid MFA method is provided during login:

$ ./test.py 
Traceback (most recent call last):
  File "/home/da/./test.py", line 8, in <module>
    need_code = ft_ss.login()
                ^^^^^^^^^^^^^
  File "/home/da/.local/lib/python3.12/site-packages/firstrade/account.py", line 134, in login
    need_code = self._handle_mfa()
                ^^^^^^^^^^^^^^^^^^
  File "/home/da/.local/lib/python3.12/site-packages/firstrade/account.py", line 266, in _handle_mfa
    raise LoginError(
firstrade.exceptions.LoginError: MFA required but no valid MFA method was provided (pin, email/phone, or mfa_secret).