Fix ``mode`` parameter in :func:`~mne.minimum_norm.get_point_spread` and :func:`~mne.minimum_norm.get_cross_talk` to correctly map public mode names (``'max'``, ``'svd'``) to internal names, expose previously hidden modes ``'sum'`` and ``'maxval'`` in the public API, and raise :class:`ValueError` for invalid mode values, by :newcontrib:`Famous077`.