Fix M-r history search losing newest-first order

[mrcnski]

Issue: see PR title. Found in agent-shell.

Details

shell-maker-search-history builds its candidates from (ring-elements comint-input-ring), which is already ordered newest first, but hands them to
completing-read as a bare list. Without display-sort-function metadata, the
completion frontend applies its own order: vertico re-sorts by length and then
alphabetically, default completion sorts the *Completions* buffer
alphabetically. Recent prompts end up buried.

Repro

In any shell-maker shell with vertico at default settings, submit bbb, then
a, then cc, and hit M-r — candidates show as a, cc, bbb instead of
cc, a, bbb.

Fix

Wrap the candidates in a completion table that declares display-sort-function
and cycle-sort-function as identity, so every frontend keeps the ring order
and M-r matches comint's own newest-first history commands. A plain closure
rather than a newer metadata helper, to stay within the emacs "27.1" floor.

(delete-dups keeping the first — i.e. most recent — occurrence of a repeated
prompt is intentional and unchanged.)

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