[fix](function) Fix window_funnel missing boundary events when window overflows at datetime upper bound

[Mryange]

For timestamps near the DATETIME upper bound (e.g. 9999-12-31 23:59:58), adding the sliding window seconds in date_add_interval<SECOND> overflows the date range. Both window_funnel and window_funnel_v2 ignored the false return value, leaving the window end timestamp at the starting point, so the boundary event inside the enlarged window was missed: window=1 returned 2 while window=10 returned 1.

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[hello-stephen]

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[Mryange]

run buildall