Test new_aux_window cleanup when set-option fails

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

tests/test_tmux.py

@@ -1069,3 +1069,17 @@ async def test_new_aux_window_returns_none_when_new_window_fails(runner):
     ]
     wid = await runner.new_aux_window("nvim", "/p", ["nvim"], "nvim · p")
     assert wid is None
+
+
+@pytest.mark.asyncio
+async def test_new_aux_window_kills_window_when_set_option_fails(runner):
+    runner._exec.side_effect = [
+        (0, "0\n", ""),    # _next_window_index
+        (0, "@7\n", ""),   # new-window -> window_id @7
+        (1, "", "nope"),   # set-option fails
+        (0, "", ""),       # kill-window cleanup
+    ]
+    wid = await runner.new_aux_window("nvim", "/p", ["nvim"], "nvim · p")
+    assert wid is None
+    # the half-built window must be cleaned up by id.
+    assert runner._exec.call_args_list[-1].args == ("kill-window", "-t", "@7")