fix(wsi): fall back to fsspec for ECS thumbnails

[raylim]

Summary

• fall back to the fsspec/S3 transport used by slide reads when the pooled boto3 thumbnail read fails
• keep serving the exact published thumbnail variant URI
• add regression coverage for the fallback

Verification

• uv run --project . pytest -q (194 passed)
• local ECS reads verified for slides 2907269, 2907260, and 2910060

This is scoped to the beta thumbnail-fetch failure; no production routing or data changes.