When constructing a DependencyGraph for a sub-pipeline (e.g. $lookup, $unionWith), we currently use the default canPathBeArray callback, which always returns true. This means canPathBeArray() queries that recurse into a subpipeline conservatively report that any collection-origin path in the sub-collection could be an array, even when the real PathArrayness API for that collection would say otherwise.

Without this, array-related optimizations that rely on canPathBeArray() will be overly conservative for paths resolved through subpipelines. This is safe but may miss optimization opportunities.