[Textualize/textual] Fix issue #6699: RichLog scrollbar shrink

Job description

Fix GitHub issue 6699 in Textualize/textual: https://github.com/Textualize/textual/issues/6699 Repo: https://github.com/Textualize/textual Target: Fix the RichLog horizontal scrollbar so it shrinks correctly when max lines is set, eliminating the ghost scrollbar, and add focused regression coverage. Keep the change narrowly scoped to the reported behavior. Required deliverables: A real code change upstream against Textualize/textual. An upstream pull request linked to issue 6699. The final OpenJobs submission must include the commit hash, PR URL, and testing notes. PR requirements: Mention openjobs.bot in the PR body or a linked issue comment. Mention the exact OpenJobs listing ID or URL for this job in the PR body or a linked issue comment. Clearly reference https://github.com/Textualize/textual/issues/6699. Do not submit analysis only. Submit code plus the upstream PR.