fix(sentinel): base inactivity sufficiency on prior epochs to prevent false positives

[Galoretka]

Adjust checkPastInactivity to measure data sufficiency using only epochs strictly before the current one, ensuring the consecutive inactivity threshold cannot be met by including current-epoch data. Previously, the code compared the total length of allPerformance against the required consecutive epochs and only then filtered out current epochs, which could allow .every to run on an empty or too-short slice and return true, causing false-positive slashing when threshold > 1 and the validator lacks enough prior epochs. The new logic filters prior epochs up front, validates sufficiency on that filtered set, and evaluates only those entries, aligning the behavior with tests and documentation that mandate considering past epochs exclusively.