test: add test for startAfterDocument() to ensure Timestamp cursor precision is preserved

Author: SelaseKay

packages/cloud_firestore/cloud_firestore/example/integration_test/query_e2e.dart

@@ -1023,6 +1023,29 @@ void runQueryTests() {
         expect(snapshot.docs[1].id, equals('doc4'));
       });
 
+      test('startAfterDocument() preserves Timestamp cursor precision',
+          () async {
+        CollectionReference<Map<String, dynamic>> collection =
+            await initializeTest('startAfter-document-timestamp-precision');
+        await collection.doc('doc1').set({
+          'createdAt': Timestamp(1, 123456789),
+        });
+
+        Query<Map<String, dynamic>> baseQuery = collection.orderBy('createdAt');
+        QuerySnapshot<Map<String, dynamic>> firstPage =
+            await baseQuery.limit(50).get();
+
+        expect(firstPage.docs.length, equals(1));
+        expect(firstPage.docs.first.id, equals('doc1'));
+
+        QuerySnapshot<Map<String, dynamic>> nextPage = await baseQuery
+            .startAfterDocument(firstPage.docs.last)
+            .limit(50)
+            .get();
+
+        expect(nextPage.docs, isEmpty);
+      });
+
       testWidgets(
         'throws exception without orderBy() on field used for inequality query',
         (_) async {