test: validate migrated thumbnail backends

[uermel]

Summary

• add deterministic thumbnail reads through real copick local, S3-compatible, and ML Croissant entities
• run the four-worker concurrency regression on the deterministic local fixture in ordinary test CI
• retain opt-in real SSH coverage for consecutive reads, a closed connection, a held lazy array, recovery, and concurrent workers
• retain bounded portal and exact published core-fixture checks as opt-in integration tests
• keep the local thumbnail benchmark as diagnostic data only; it is not treated as proof of bounded reads
• keep retry and reconnect behavior owned by the storage/backend layer rather than shared UI
• remove the unsolicited ecosystem-validation workflow and the always-skipped generic-backend harness

Stack

This is stack layer 3 of 4. It targets uermel/zarr-v3-p1-runtime-thumbnail.

Validation

• full integration-capable suite with the deterministic local S3 service: 39 passed, 3 opt-in skips
• regular-CI concurrency test: exactly four callbacks, one successful result for each of four workers
• real SSH held-array/post-drop/concurrent-worker test: passed
• live portal dataset 10301, run 14077: (256, 256) uint8, range 0–255
• clean wheel environment: copick 2.0.0-alpha.1 and Zarr 3.1.6 selected
• Ruff, Black, pre-commit, and remaining workflow validation: passed

This PR is part of #51, but does not close it. Remote legacy-v2 coverage, the complete cold/warm/cache/layout matrix, and downstream napari/ChimeraX alpha smoke evidence remain tracked there.

Part of #51
Part of #53