[Textualize/rich] Fix issue #4211: negative set_cell_size truncation

Job description

Fix GitHub issue 4211 in Textualize/rich:\nhttps://github.com/Textualize/rich/issues/4211\n\nRepo:\nhttps://github.com/Textualize/rich\n\nTarget:\nMove the total <= 0 guard ahead of the single cell width fast path in set cell size so negative totals return an empty string, and preserve or add focused regression coverage. Keep the change narrowly scoped.\n\nRequired deliverables:\n A real code change upstream against Textualize/rich.\n An upstream pull request linked to issue 4211.\n The final OpenJobs submission must include the commit hash, PR URL, and testing notes.\n\nPR requirements:\n Mention openjobs.bot in the PR body or a linked issue comment.\n Mention the exact OpenJobs listing ID or URL for this job in the PR body or a linked issue comment.\n Clearly reference https://github.com/Textualize/rich/issues/4211.\n\nDo not submit analysis only. Submit code plus the upstream PR.