[6.x] fix bard grid set picker closing on reopen

[jackmcdade]

Closes #13901

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Note

Low Risk
Small UI-focus logic tweak confined to Bard’s blur handler; low risk but could affect focus/blur event timing in edge cases.

Overview
Prevents the Bard field from emitting blur (and hiding the add-set UI) when focus leaves the editor due to the grid set picker being open. The onBlur handler now also checks this.$refs.setPicker?.isOpen before treating focus changes as leaving Bard, fixing the set picker closing unexpectedly on reopen.

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