Add test cases for `where.not` with polymorphic association

`where.not` with multiple conditions is still unexpected behavior. But
`where.not` with only polymorphic association has already been fixed in
213796fb.

Closes #14161.
Closes #16983.
Closes #17010.
Closes #26207.

activerecord/test/cases/relation/where_test.rb

@@ -109,6 +109,15 @@ def test_polymorphic_shallow_where
       assert_equal expected.to_sql, actual.to_sql
     end
 
+    def test_polymorphic_shallow_where_not
+      treasure = treasures(:sapphire)
+
+      expected = [price_estimates(:diamond), price_estimates(:honda)]
+      actual   = PriceEstimate.where.not(estimate_of: treasure)
+
+      assert_equal expected.sort_by(&:id), actual.sort_by(&:id)
+    end
+
     def test_polymorphic_nested_array_where
       treasure = Treasure.new
       treasure.id = 1
@@ -121,6 +130,16 @@ def test_polymorphic_nested_array_where
       assert_equal expected.to_sql, actual.to_sql
     end
 
+    def test_polymorphic_nested_array_where_not
+      treasure = treasures(:diamond)
+      car = cars(:honda)
+
+      expected = [price_estimates(:sapphire_1), price_estimates(:sapphire_2)]
+      actual   = PriceEstimate.where.not(estimate_of: [treasure, car])
+
+      assert_equal expected.sort_by(&:id), actual.sort_by(&:id)
+    end
+
     def test_polymorphic_array_where_multiple_types
       treasure_1 = treasures(:diamond)
       treasure_2 = treasures(:sapphire)