test: assert recurring-events window against calendar-sync's own range

The rolling-window integration test hand-rolled its bound as now minus 90 days to now plus 365 days at the current clock time. The code's window is minus-3 / plus-12 calendar months at day granularity. So a boundary occurrence stamped at midnight sat inside the code's window but before the now-minus-90-days bound, and the test passed or failed by the day it ran (green 2026-07-16, red 2026-07-17).

I switched the assertion to calendar-sync--get-date-range, so the test checks the invariant it names: every emitted occurrence lands within the window the pipeline computed, stable on any date.

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