Check the skipped integration case at points, not symbolically

The difference between the derivative and the integrand is zero, but Simplify does
not reduce it to zero, so a symbolic assertion would keep failing after the answer
became right. Checked at three points instead, and the skip reason now says what
actually makes it pass: this fix together with the integration-recursion one.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

Author: Rafael-SOWNet

Sources/Tests/UnitTests/Calculus/IntegrationTest.cs

@@ -84,15 +84,26 @@ public void TestPowerSubstitution(string initial, string expected)
         //   3 * x2 * (x3 + 2) ^ 2  ->  C + -5/4 * x^6 + (x^3 + 2) * (x^6 / 2 + 2 * x^3)
         // which is not an antiderivative of the integrand: at x = 2 the integrand is 1200
         // and the derivative of that answer is 1536. The true antiderivative is
-        // (x^3 + 2)^3 / 3, which differentiates back exactly. The old expectation was
-        // pinning the wrong answer, so it is asserted here the way it should be and left
-        // skipped until integration produces it.
-        [Fact(Skip = "Integration returns a wrong antiderivative here; see the comment above")]
+        // (x^3 + 2)^3 / 3. The old expectation was pinning the wrong answer, so the case
+        // is asserted here the way it should be.
+        //
+        // Checked at points rather than symbolically: the difference is zero, but Simplify
+        // does not reduce it to zero, so a symbolic assertion would fail even once the
+        // answer is right.
+        [Fact(Skip = "Integration returns a wrong antiderivative here on its own; " +
+                     "it is right once the integration-recursion fix is also applied")]
         public void PowerSubstitutionIsAnAntiderivative()
         {
             var integrand = "3 * x2 * (x3 + 2) ^ 2".ToEntity();
-            var residual = (integrand.Integrate("x").Differentiate("x") - integrand).Simplify();
-            Assert.Equal(Entity.Number.Integer.Create(0), residual);
+            var derivative = integrand.Integrate("x").Differentiate("x");
+            foreach (var point in new[] { 2, 3, 5 })
+            {
+                var expected = integrand.Substitute(x, point).EvalNumerical();
+                var actual = derivative.Substitute(x, point).Substitute("C", 0).EvalNumerical();
+                Assert.True((expected - actual).Abs().EDecimal.ToDouble() < 1e-9,
+                    $"at x = {point} the integrand is {expected.Stringize()} "
+                    + $"but the derivative of the answer is {actual.Stringize()}");
+            }
         }
 
         [Theory]