test: update existing tests for identity-first APIs and new lock constants

Update refresh and harness mocks to return real Response objects so the new response-inspection paths exercise the same API surface as production fetch calls.

Persist mocked account storage across load/save cycles in index tests so syncActiveIndexFromDisk and debounced save behavior preserve failover state between requests.

Align billing-header regression expectations with the current version-aware hash algorithm and record the T40 evidence/learnings for the full-suite gate.