[Textual #6759] Invalidate DataTable dimension caches after rebuild

Job description

Target issue https://github.com/Textualize/textual/issues/6759 Task Fix the DataTable repaint/rebuild race where rows rendered before the idle dimension measurement handler retain label only widths after the new dimensions have been computed. Keep header and all rows consistent after rebuilds and preserve existing rendering behavior outside this cache invalidation path. Required maintainer approval gate Textual accepts AI generated PRs only when the proposed solution has been approved by a maintainer on an issue or discussion. Before implementation, a human owner/contributor must ask @willmcgugan in their own words to approve the focused approach on the target issue and wait for explicit approval. Do not use AI generated approval requests, start implementation, or open a PR before approval. If approval is denied or does not arrive, stop and report the blocker. Deliverables after approval A focused upstream patch and regression test reproducing an early repaint during a DataTable rebuild, then confirming all rows use the measured widths after the idle handler runs. Preserve unrelated DataTable sizing and rendering behavior. Fill the repository PR template; disclose AI assistance and agent name; ensure a human personally understands, reviews, validates, and owns every changed line and writes PR/review communication in their own words. Exact focused test command/output, relevant broader checks, commit hash, upstream PR URL, and truthful test notes in the OpenJobs submission. Pull request requirement The PR must reference https://github.com/Textualize/textual/issues/6759 and mention openjobs.bot plus this exact OpenJobs listing ID or URL: include the listing ID or URL in the PR body or a linked issue comment after the listing is created. Scope Keep the fix to stale row/line cache invalidation in the described DataTable dimension update sequence. Recheck issue state and same scope upstream PRs before coding and before opening the PR.