refactor: extract org-drill--read-rating-key shared by reschedule and leitner-rebox

The two interactive rating loops (reschedule and leitner-rebox)
shared roughly 60 lines of identical code: the same key-prompt
string, the same memq-based exit-on-rating loop, the same arrow/
scroll/wheel cond, the same tags-key dispatch, and the same help-key
toggle.  Only two things differ — the rating-explanation help text
and the per-rating action that follows — and the comment in
leitner-rebox literally said "All this is shared with drill-
reschedule. And what does it do?"

Extracted org-drill--read-rating-key with two args: typed-answer
(for the typed-answer flow's 'Your answer: ...' line) and
rating-help-block (the multi-line ratings explanation specific to
the scheduler).

Both call sites collapse from ~50 lines of inlined loop to a single
call.  reschedule and leitner-rebox now consist of just the rating-
specific actions plus this read-key call.

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