[mypyc] Fix unsafe borrowing of instance attributes with free-threading (#21688)

Don't borrow native attributes any more, except for final native
attributes (these can't be rebound, so they are safe to borrow), and
attributes with `vec` types (`vec` requires explicit synchronization by
design).

Work on mypyc/mypyc#1203.

I used coding agent assist for this, but manually reviewed changes.

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>