Fix admin-transfer test coverage and remove ineffective auth assertions

[Copilot]

This PR addresses the review feedback on the new transfer_admin tests in payment-distributor. The previous unauthorized-path test did not validate authorization behavior and included unused variables; the chain test comment/flow also did not reflect what was actually being asserted.

• transfer_admin unauthorized-path test corrected

  • Reworked test_transfer_admin_fails_if_unauthorized to assert the contract’s authorization guard directly instead of relying on mock_all_auths() behavior.
  • Removed unused locals introduced in the original version (unauthorized_caller, intermediate result usage that was not asserted).

• Admin-chain test assertion flow fixed

  • Updated test_transfer_admin_chain to execute the second transfer step and assert state transition to new_admin_2.
  • Reworded the misleading inline comment so the test intent matches actual behavior.

let result = distributor.try_transfer_admin(&new_admin);
assert_eq!(result, Err(Ok(Error::Unauthorized)));
assert_eq!(distributor.get_admin().unwrap(), original_admin);