fix(Accordion): leaking stale summaryButtons refs on panel removal

[zigzagdev]

Fixes #

Button for Accordion's addSummaryButton only pushed into this.summaryButtons and never removed an entry when a panel's button unmounted (its ref callback fires with null, which was ignored). After removing a panel, a stale detached DOM node stayed in the array, so keyboard navigation could focus that stale node instead of the correct remaining panel.
With this fix, keying summaryButtons by the child's render index and clearing the slot when its ref detaches, and added a regression test.

Additional description

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