Keep keyboard focus in the answer dialog

After a wrong answer the reopened dialog had no keyboard focus. The dialog is
lifted and focus is forced onto it and its entry each time it opens.

Task: MIL-003#8

tests/test_turtle_screen.py

@@ -0,0 +1,70 @@
+"""Tests of the display-free dialog focus helpers in `turtle_screen`."""
+
+from collections.abc import Callable, Sequence
+
+from us_states_game.turtle_screen import DialogWidget, find_widgets, focus_dialog
+
+
+class FakeWidget:
+    """A Tk widget stand-in that records the calls made on it."""
+
+    def __init__(self, tk_class: str, children: Sequence["FakeWidget"] = ()) -> None:
+        self.tk_class = tk_class
+        self.children = list(children)
+        self.calls: list[str] = []
+
+    def winfo_class(self) -> str:
+        return self.tk_class
+
+    def winfo_children(self) -> Sequence[DialogWidget]:
+        return self.children
+
+    def after(
+        self, ms: int, func: Callable[[DialogWidget], None], widget: DialogWidget
+    ) -> str:
+        return ""
+
+    def lift(self) -> None:
+        self.calls.append("lift")
+
+    def focus_force(self) -> None:
+        self.calls.append("focus_force")
+
+
+def test_find_widgets_returns_nested_matches_in_order() -> None:
+    entry = FakeWidget("Entry")
+    frame = FakeWidget("Frame", [entry])
+    dialog = FakeWidget("Toplevel", [frame])
+    root = FakeWidget("Tk", [FakeWidget("Canvas"), dialog])
+
+    assert find_widgets(root, ("Entry", "TEntry")) == [entry]
+    assert find_widgets(root, "Toplevel") == [dialog]
+
+
+def test_focus_dialog_lifts_dialog_and_focuses_entry() -> None:
+    entry = FakeWidget("Entry")
+    dialog = FakeWidget("Toplevel", [FakeWidget("Frame", [entry])])
+    root = FakeWidget("Tk", [dialog])
+
+    focus_dialog(root)
+
+    assert dialog.calls == ["lift", "focus_force"]
+    assert entry.calls == ["focus_force"]
+
+
+def test_focus_dialog_uses_newest_dialog() -> None:
+    old = FakeWidget("Toplevel")
+    new = FakeWidget("Toplevel")
+
+    focus_dialog(FakeWidget("Tk", [old, new]))
+
+    assert old.calls == []
+    assert new.calls == ["lift", "focus_force"]
+
+
+def test_focus_dialog_does_nothing_without_dialog() -> None:
+    canvas = FakeWidget("Canvas")
+
+    focus_dialog(FakeWidget("Tk", [canvas]))
+
+    assert canvas.calls == []