Closing a pane contributed by a plugin used to disable the entire plugin,
unloading every one of its contributions. For a plugin that owns several
independent panes (e.g. Bot Mode's Cronjobs pane alongside its Bots roster
and composer middleware), closing one pane silently killed the rest.
Now: closing one pane of a multi-pane plugin dismisses only that pane; the
plugin stays enabled and its other panes/commands/middleware keep working.
Reset layout restores dismissed contributed panes. A single-pane plugin
keeps the existing symmetric behavior (Close disables the plugin, with
Settings -> Plugins as the recovery path).
Adds regression coverage for both cases.
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