Pre-resolved Graph credentials.
OptionalLookaheadHow far forward the FIRST poll's calendarView window extends (days). Defaults to 30.
OptionalModuleTest seam: override the module loader. Defaults to the lazy dynamic import.
OptionalNowTest seam: a fixed "now" for the first-poll window (so window math is deterministic).
Options for GraphCalendarClient. The first-poll window bounds the initial
calendarViewdelta; subsequent polls follow the opaque@odata.deltaLinkcursor.