CVE-2026-9748 $_internalConvertBucketIndexStats may crash the mongod server when working on no timeseries input

The $internalConvertBucketIndexStats stage used PauseExecution as a way to signal "skip this document" when an index stats conversion failed. But PauseExecution is not a general purpose skip mechanism, but rather a TeeBuffer-internal signal used solely by $facet to coordinate its sub-pipelines...