Support the HQL scalar COUNT subquery pattern over an embedded array via UNNEST, translating it to MongoDB's $filter + $size.

Example HQL:

SELECT c.id, (SELECT count(*) FROM UNNEST(c.lineItems) li WHERE li.qty > 4) FROM Cart c

Expected MQL:

{ "$addFields": { "_count": { "$size": { "$filter": {
    "input": "$lineItems",
    "cond": { "$gt": ["$$this.qty", 4] }
} } } } }

This shape returns the count of matching array elements as a scalar value per parent document. It maps naturally to $filter + $size without requiring $unwind.