#3559 MockVaultProvider.get and delete throw synchronously instead of rejecting

src/domain/vaults/mock_vault_provider.ts declares get(secretKey): Promise and delete(secretKey): Promise as non-async functions, and for a missing key each throws synchronously instead of returning a rejected promise. A caller that awaits the call sees no difference, but one that chains .catch() on the returned promise gets an uncaught synchronous throw instead. Every other built-in provider rejects. Found while triaging swamp-club#3557; unrelated to that work, so filed separately. Fix: return Promise.reject(...) or mark the methods async, and add a unit test that the missing-key path rejects rather than throws.