Make cluster scans best-effort with partial coverage

[lh0156]

Fixes #3411

What changed

• Make cluster scan_iter() best-effort when require_full_coverage=False.
• Skip only per-node ConnectionError and TimeoutError failures while continuing to scan reachable nodes.
• Preserve fail-fast behavior when require_full_coverage=True.
• Continue to propagate Redis command errors such as ResponseError.
• Keep synchronous and asynchronous cluster clients consistent.
• Add regression tests for both clients and the failure boundaries.

Verification

• pytest -q tests/test_cluster.py -k 'cluster_scan_iter' — 4 passed
• pytest -q tests/test_asyncio/test_cluster.py -k 'cluster_scan_iter' — 5 passed
• invoke linters
• ruff format --check
• git diff --check
• python -m compileall

The full Redis Cluster integration suite requires local Redis cluster services, which are not available in this workspace.