FIX: correct mode parameter in get_point_spread and get_cross_talk

[Famous077]

Reference issue (if any)

Fixed issue #13128

What does this implement/fix?

Found a bug where the mode parameter in get_point_spread() and
get_cross_talk() wasn't working as documented. The public API says
'max' and 'svd' are valid modes, but the internal helper function
_summarise_psf_ctf uses different names ('maxnorm' and 'pca'),
so passing 'max' or 'svd' just silently returned all vertices
without any summarisation — no error, no warning.

Changes made:

• added validation so invalid mode values now raise a ValueError
• map public names to internal ones before calling _summarise_psf_ctf
• updated existing tests to use the correct public mode names

Additional information

Verified the fix works correctly with both individual vertex indices and labels.
All 3 existing tests pass after the changes.