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_dashed_line.py

@@ -0,0 +1,76 @@
+import pytest
+
+from tests.fakes import FakePen
+from turtle_challenges.constants import DASH_COUNT, DASH_LENGTH, GAP_LENGTH
+from turtle_challenges.dashed_line import draw_dashed_line
+
+
+def test_draw_dashed_line_alternates_dash_and_gap() -> None:
+    pen = FakePen()
+
+    draw_dashed_line(pen, dash_count=2)
+
+    assert (
+        pen.calls[:-1]
+        == [
+            ("pendown", ()),
+            ("forward", (DASH_LENGTH,)),
+            ("penup", ()),
+            ("forward", (GAP_LENGTH,)),
+        ]
+        * 2
+    )
+
+
+def test_draw_dashed_line_draws_the_default_number_of_dashes() -> None:
+    pen = FakePen()
+
+    draw_dashed_line(pen)
+
+    assert pen.names().count("pendown") == DASH_COUNT + 1
+    assert pen.names().count("forward") == 2 * DASH_COUNT
+
+
+def test_draw_dashed_line_leaves_the_pen_down() -> None:
+    pen = FakePen()
+
+    draw_dashed_line(pen, dash_count=3)
+
+    assert pen.calls[-1] == ("pendown", ())
+
+
+def test_draw_dashed_line_uses_the_given_lengths() -> None:
+    pen = FakePen()
+
+    draw_dashed_line(pen, dash_count=1, dash_length=4, gap_length=7)
+
+    assert pen.args_of("forward") == [(4,), (7,)]
+
+
+def test_draw_dashed_line_with_no_dashes_only_puts_the_pen_down() -> None:
+    pen = FakePen()
+
+    draw_dashed_line(pen, dash_count=0)
+
+    assert pen.names() == ["pendown"]
+
+
+def test_draw_dashed_line_rejects_a_negative_count() -> None:
+    pen = FakePen()
+
+    with pytest.raises(ValueError, match="dash_count"):
+        draw_dashed_line(pen, dash_count=-1)
+
+    assert pen.calls == []
+
+
+@pytest.mark.parametrize(("dash_length", "gap_length"), [(0, 10), (10, 0), (-1, 10)])
+def test_draw_dashed_line_rejects_a_length_that_is_not_positive(
+    dash_length: float, gap_length: float
+) -> None:
+    pen = FakePen()
+
+    with pytest.raises(ValueError, match="lengths"):
+        draw_dashed_line(pen, dash_length=dash_length, gap_length=gap_length)
+
+    assert pen.calls == []