fix: replace xargs trim with sed to handle quotes in descriptions (#2351)

xargs re-parses stdin as shell tokens, causing 'unterminated quote'
errors when feature descriptions contain apostrophes, double quotes,
or backslashes. Replace with sed-based whitespace trim that preserves
input verbatim.

Add regression tests for special characters in descriptions (core and
extension scripts), plus a negative test for whitespace-only input.

Fixes #2339