Add draw_square and draw_dashed_line with tests

Challenge 1 draws four sides and turns right 90 degrees after each. Challenge 2 puts
the pen down for a dash and up for a gap and leaves it down at the end. Lengths and
counts come from constants.py; invalid values raise ValueError.

Task: MIL-002#3
Task: MIL-002#4
Task: MIL-002#5

tests/test_square.py

@@ -0,0 +1,46 @@
+import pytest
+
+from tests.fakes import FakePen
+from turtle_challenges.constants import RIGHT_ANGLE_DEGREES, SQUARE_SIDE_LENGTH
+from turtle_challenges.square import draw_square
+
+
+def test_draw_square_moves_forward_and_turns_right_four_times() -> None:
+    pen = FakePen()
+
+    draw_square(pen)
+
+    assert (
+        pen.calls
+        == [
+            ("forward", (SQUARE_SIDE_LENGTH,)),
+            ("right", (RIGHT_ANGLE_DEGREES,)),
+        ]
+        * 4
+    )
+
+
+def test_draw_square_uses_the_given_side_length() -> None:
+    pen = FakePen()
+
+    draw_square(pen, side_length=40)
+
+    assert pen.args_of("forward") == [(40,)] * 4
+
+
+def test_draw_square_returns_the_turtle_to_its_start_heading() -> None:
+    pen = FakePen()
+
+    draw_square(pen)
+
+    assert pen.heading() == 0
+
+
+@pytest.mark.parametrize("side_length", [0, -5])
+def test_draw_square_rejects_a_side_that_is_not_positive(side_length: float) -> None:
+    pen = FakePen()
+
+    with pytest.raises(ValueError, match="side_length"):
+        draw_square(pen, side_length=side_length)
+
+    assert pen.calls == []