fix(wsi): isolate and repair slide block caches

[raylim]

Summary

• isolate fsspec slide block caches per source, worker, and block size
• pass the configured block size at file-open time and repair stale/corrupt slide handles once
• retry transient source and thumbnail reads with bounded backoff and expose repair/error metrics

Verification

• uv run pytest -q (210 passed)
• final container health/readiness and authenticated tile/thumbnail smoke checks passed against ECS-backed data

This is intended for the beta WSI tile service first; production rollout remains a separate deployment decision.