Harden native gate thread against affinity mask growth (#59132)

Harden native gate thread against affinity mask growth (#59132)

Hardened ThreadpoolMgr::GateThreadStart against the possibility that the observed group-local affinity mask contains set bits at positions higher than the total group-local CPU count that was captured during earlier initialization.

This fixes customer-reported crashes which have occurred on multi-group machines with heterogenous CPU counts across groups (but the same crash can probably also occur on single-group VMs if CPUs are hot-added and are then manually added to the process affinity mask).