from abc import ABC, abstractmethod


class MailIntakeAuthProvider(ABC):
    """
    Abstract authentication provider.

    Mail adapters depend on this interface, not on concrete
    OAuth or credential implementations.
    """

    @abstractmethod
    def get_credentials(self):
        """
        Return provider-specific credentials object.

        This method is synchronous by design and must either
        return valid credentials or raise MailIntakeAuthError.
        """
        raise NotImplementedError