Sink integration tests are copy-pasted skeletons: extract one shared runner #433

Severity: Low (test maintenance cost)

Evidence

• find internal/sink -name '*integration_test.go' → 13 files, 3,973 lines at HEAD.
• The skeletons are copy-pasted: the first 60 lines of s3/export_integration_test.go and gcs/export_integration_test.go differ only in test name and constants.

Recommended fix

One table-driven shared integration runner (container setup, export, read back, delete) parameterised per backend.

Source: consolidated multi-model review (2026-10-04, verified at 34cfeaa4); re-verified against main @ e794bb67 (after #413).