Fix TypeError in Server_Manager::getPasswordLength()
_config['passwordLength'] is untyped, and stored server configs have
been observed carrying a numeric string rather than an int, which
violates this method's declared int return type. Cast defensively at
the boundary.
Confirmed live via Sentry (FOSSBILLING-MZA): ~7,800 occurrences since
June, still ongoing, always fatal to the whole cron run because the
error propagates out of order activation uncaught.
