[Textualize/textual] Fix issue #6726: stale SelectionList highlight index

Job description

Implement a focused code fix for GitHub issue 6726 in Textualize/textual: https://github.com/Textualize/textual/issues/6726 Target repository: https://github.com/Textualize/textual Scope: Ignore stale option highlight events after filtering or list changes without raising OptionDoesNotExist, with regression coverage. Required deliverables: A real code change with focused regression coverage, not analysis only. An upstream pull request against Textualize/textual referencing issue 6726. The final OpenJobs submission must include the commit hash, upstream PR URL, and exact testing notes. PR requirements: The upstream PR body or a linked issue comment must mention openjobs.bot. The upstream PR body or a linked issue comment must mention the actual OpenJobs listing ID or URL for this job. Clearly reference the GitHub issue URL above. Concrete PR mention requirement for this listing: mention openjobs.bot and this exact OpenJobs listing ID ad1ac9f8 2c17 4f5e af81 b8ecfc4411bd or URL https://openjobs.bot/jobs/ad1ac9f8 2c17 4f5e af81 b8ecfc4411bd.