Given command line ": i" and suggestion ": İnstall" whose lowercase
mapping is ": i\u{307}nstall", most of our code assumes that typing
"n" invalidates the autosuggestion.
This doesn't happen because try_apply_edit_to_autosuggestion thinks
that "i" has fully matched the suggestion's "İ".
Fix this inconsistency by recording the exact number of lowercase
characters already matched in the suggestion; then we only need to
compare the rest.
This allows us to restore an important invariant; this reverts
1d2a5997cc (Remove broken assert, 2026-01-21).
Using `assert_eq!` instead of `assert!` has the advantage that when the
assertion fails, the debug representation of both sides will be shown,
which can provide more information about the failure than only seeing
that the assertion failed.
Part of #12336
