Fix repeated tab causing repeated smartcase completion insertion

Doing

	firefox --pro TAB TAB TAB

results in

	firefox --profile --ProfileManager

git-bisect points to 3546ffa3ef5 (reader
handle_completions(): remove dead filtering code, 2026-01-02)
but that regression has already been fixed by 85e76ba3561
(Fix option substr completions not being filtered out, 2026-04-16).

However in between those two commits, the above case has also been
broken by 2f6b1eaaf9c (reader handle_completions(): don't consider
odd replacing completions for common prefix, 2026-01-02)

The first TAB inserts "--profile ", including the trailing space.
However it also shows the completion pager, which means that subsequent
TABs will insert after the space.

The trailing space does not make sense unless we navigate the pager.
Remove it in all cases, to fix this smartcase scenario.

Alternatively, we could start navigating the pager in this case
(and keep the trailing space), but that's probably too inconsistent.