xbps: include `stdout` and `stderr` in module output (#12234)
* fix(xbps): add stdout/stderr to module output and fix error message typo
Include stdout and stderr from the last executed command in all
exit_json and fail_json calls so users can see the actual xbps output
when debugging failures (addresses issue #2478).
* test(xbps): add basic unit tests using uthelper
Cover install (new, already present, failure) and remove (installed,
absent) scenarios. Verifies stdout/stderr are propagated in output.
* changelog: add fragment for PR 12234
* Add version_added.
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(cherry picked from commit 7dc2441fc8)
Co-authored-by: Alexei Znamensky <103110+russoz@users.noreply.github.com>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>
Co-authored-by: Felix Fontein <felix@fontein.de>
