Blank (or whitespace-only) answers to the home-channel prompt in the
interactive setup wizards previously left any previously saved
*_HOME_CHANNEL / *_HOME_ROOM env value in place, so operators could not
clear a stale home channel by re-running setup. Strip the prompt input
and call remove_env_value() on blank answers across the Discord, Slack,
Feishu, Matrix, Mattermost, WeCom and WhatsApp plugin setup wizards,
with per-adapter wizard tests covering set/clear/whitespace flows.
Squash of the three commits from PR #58421 (setup-wizard fix, matrix/
wecom extension, and 6-adapter test coverage) — one commit per
contributor on this salvage branch.
Fixes#12423
Salvaged from #58421
