Provide Link authority to checkpoint fixtures

[SaladDay]

Three integration gateway fixtures omitted Sandbox Link authority. Environment binding now requires that authority before initialization or checkpoint work, so these fixtures stopped before reaching the behavior they tested.

Supply the existing database-backed Link authority in each fixture, matching production wiring. No production code or test assertions change.

Validation: the original four checkpoint failures reproduced before the fix and passed afterward. All 398 integration tests across the three CI shards passed on the committed tree, including every initialization and suspend/wake failure from #572. Integration vet, formatting, naming, documentation and CI checks passed; coordinator reviewed the complete three-line change.

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