style(tui): apply ruff format to tdd_slash_overlay_keyboard_nav_steps

Remove unnecessary parentheses around f-string assert messages to
satisfy ruff format --check in CI lint job.

features/steps/tdd_slash_overlay_keyboard_nav_steps.py

@@ -93,9 +93,7 @@ def step_has_selected_index(context: object) -> None:
 def step_selected_index_equals(context: object, expected: int) -> None:
     """Verify the overlay selected_index equals the expected value."""
     actual = context.overlay.selected_index
-    assert actual == expected, (
-        f"Expected selected_index={expected}, got {actual}"
-    )
+    assert actual == expected, f"Expected selected_index={expected}, got {actual}"
 
 
 @then("the overlay selected_index should not exceed the command count")
@@ -103,9 +101,7 @@ def step_selected_index_bounded(context: object) -> None:
     """Verify selected_index does not exceed the number of commands."""
     count = len(context.test_commands)
     actual = context.overlay.selected_index
-    assert actual < count, (
-        f"selected_index={actual} must be < command count={count}"
-    )
+    assert actual < count, f"selected_index={actual} must be < command count={count}"
 
 
 @then("the selected command should be the second command in the list")