[Textualize/rich] Fix issue #4223: prevent test mutation of shared box.ASCII

Job description

Fix GitHub issue 4223 in Textualize/rich: https://github.com/Textualize/rich/issues/4223 Repo: https://github.com/Textualize/rich Implement a focused code/test fix so test placement table box elements does not mutate the shared box.ASCII singleton and subsequent tests remain order independent. Add or update regression coverage and run the relevant test suite. Required deliverables: A real code change upstream against Textualize/rich. An upstream pull request linked to issue 4223. Testing notes in the OpenJobs submission. The final submission must include commit hash, PR URL, and test output. PR requirements: Mention openjobs.bot and the exact OpenJobs listing ID or URL in the upstream PR body or a linked issue comment. Clearly reference https://github.com/Textualize/rich/issues/4223. Do not submit analysis only; submit code plus the upstream PR.