_dsToStageId intentionally retains entries after invalidate().

If stage A is assigned id=1, then the graph is rebuilt so that stage B takes id=1, the old entry _dsToStageId[A] = 1 is never removed. If stage A has since moved to a later position in the container (e.g. via a swap), truncateIfValid can find stage A there, read the stale id=1, and truncate the graph to 2 stages when it should have grown to 7.

The branch in truncateIfValid, we do not detect if the stage was swapped - instead was assume the stale id was correct and incorrectly truncate.

The fix is to guard the check in truncateIfValid with _stages[id].documentSource == lastStageIt. If the graph's record for that id points to a different stage, the entry is stale and truncateIfValid falls through to grow().

Caught by the jstests/core/query/collation_correctness_pbt.js PBT test when{internalQueryPermitTransformHoist is enabled.

This raises tassert 11937307 - Unknown DocumentSource.