fix(db-mongodb): hasNextPage with polymorphic joins (#11394)

Previously, `hasNextPage` was working incorrectly with polymorphic joins
(that have an array of `collection`) in MongoDB.

This PR fixes it and adds extra assertions to the polymorphic joins
test.

---------

Co-authored-by: Jarrod Flesch <jarrodmflesch@gmail.com>

test/joins/int.spec.ts

@@ -1205,6 +1205,37 @@ describe('Joins Field', () => {
       expect(parent.children.docs[1]?.value.id).toBe(child_1.id)
       expect(parent.children.docs[1]?.relationTo).toBe('multiple-collections-1')
 
+      // Pagination across collections
+      parent = await payload.findByID({
+        collection: 'multiple-collections-parents',
+        id: parent.id,
+        depth: 1,
+        joins: {
+          children: {
+            limit: 1,
+            sort: 'title',
+          },
+        },
+      })
+
+      expect(parent.children.docs).toHaveLength(1)
+      expect(parent.children?.hasNextPage).toBe(true)
+
+      parent = await payload.findByID({
+        collection: 'multiple-collections-parents',
+        id: parent.id,
+        depth: 1,
+        joins: {
+          children: {
+            limit: 2,
+            sort: 'title',
+          },
+        },
+      })
+
+      expect(parent.children.docs).toHaveLength(2)
+      expect(parent.children?.hasNextPage).toBe(false)
+
       // Sorting across collections
       parent = await payload.findByID({
         collection: 'multiple-collections-parents',