Fix headless bar widget test skip

[almondsun]

Summary

• make optional temporary-directory cleanup a successful no-op when the directory was never created
• preserve successful exit status for the headless bar widget contract skip
• allow the aggregate shell suite to continue past that skip

Fixes #6337.

Root cause

The EXIT trap returned the status of a final [[ ... ]] && rm ... expression. On the early headless skip path, TMPDIR is unset, so cleanup returned 1 and overrode the explicit exit 0.

Validation

Passed:

• env -u WAYLAND_DISPLAY bash test/shell.d/bar-widget-contract-test.sh
• bash -n test/shell.d/bar-widget-contract-test.sh
• git diff --check
• ./test/cli (as part of ./test/all)

Aggregate limitation:

• ./test/shell and ./test/all both continue beyond the repaired test, then stop at the unrelated existing bin-style-test.sh check because bin/omarchy-display-text-size uses raw command -v calls. This file is unchanged by the PR.

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