The $_internalValidateArrayness stage was introduced in SERVER-125083 to validate the dependency graph's arrayness analysis. While the stage is internal, it should still function correctly when used against timeseries collections, and we currently lack test coverage confirming this.

agg_stage_coverage.js is the standard location for verifying that aggregation stages behave correctly on timeseries collections. We should add a test case there for $_internalValidateArrayness to:

Confirm the stage can be used in a pipeline against a timeseries collection without unexpected errors.
Verify scalar field paths (including measurement and metaField paths) pass validation.
Verify that fields known to be arrays (e.g. measurements that are arrays) cause the stage to fail with the expected error code (12508302).
Cover both the explain path (stage should not be auto-injected) and normal execution path.