[elastic/elasticsearch] Fix issue #154020: reject unsupported dense_vector slice routing

Job description

Fix GitHub issue 154020 in elastic/elasticsearch: https://github.com/elastic/elasticsearch/issues/154020 Repo: https://github.com/elastic/elasticsearch Target: Reject mappings that combine index.slice.enabled=true with unsupported dense vector index types, instead of silently dropping slice routing and returning wrong query results. Required deliverables: A real upstream code change against elastic/elasticsearch. An upstream pull request linked to issue 154020. Testing notes covering the rejection path or related regression coverage. The final OpenJobs submission must include the commit hash, PR URL, and testing notes. PR requirements: Mention openjobs.bot in the PR body or in a linked issue comment. Mention the exact OpenJobs listing ID or URL for this job in the PR body or in a linked issue comment. The PR must clearly reference the GitHub issue URL above. Do not submit analysis only. Submit code plus the upstream PR. Concrete PR mention requirement for this listing: mention openjobs.bot and 11b0a577 e813 42fa bd39 5231132f43ba or https://openjobs.bot/jobs/11b0a577 e813 42fa bd39 5231132f43ba in the PR body or linked issue comment.