Add more windowSliding() tests (#50)

Before:
<img width="845" alt="image"
src="https://github.com/user-attachments/assets/a5e46f5d-32eb-4e92-a308-fbbf884e48e8">
After:
<img width="845" alt="image"
src="https://github.com/user-attachments/assets/8dfbaf1d-e9b8-4a43-b4cc-0579d372e194">

src/test/java/com/pivovarit/gatherers/blackbox/WindowSlidingTest.java

@@ -25,11 +25,24 @@ void shouldRejectInvalidStep() {
           .hasMessage("'step' must be greater than or equal to zero");
     }
 
+    @Test
+    void shouldRejectInvalidWindowSizeAndStep() {
+        assertThatThrownBy(() -> MoreGatherers.windowSliding(3, 4))
+          .isInstanceOf(IllegalArgumentException.class)
+          .hasMessage("'step' must be less than or equal to 'windowSize'");
+    }
+
     @Test
     void shouldWindowSlidingEmpty() {
         assertThat(Stream.empty().gather(MoreGatherers.windowSliding(2, 1))).isEmpty();
     }
 
+    @Test
+    void shouldWindowSlidingWithWindowSizeGreaterThanStreamSize() {
+        assertThat(Stream.of(1, 2, 3).gather(MoreGatherers.windowSliding(4, 1)))
+          .containsExactly(of(1, 2, 3));
+    }
+
     @Test
     void shouldWindowSlidingWithStep1() {
         assertThat(Stream.of(1, 2, 3, 4, 5).gather(MoreGatherers.windowSliding(2, 1)))