Fix transfer_admin tests to validate auth behavior and chained state transitions

[Copilot]

This addresses the review-thread feedback on PR #273 by fixing the transfer_admin test cases that were not asserting the intended behavior. The update is scoped to the commented tests only, with no contract logic changes.

• test_transfer_admin_fails_if_unauthorized now validates auth failure correctly

  • Removed env.mock_all_auths() from this test (which previously bypassed require_auth()).
  • Replaced unused variables / no-op flow with explicit mock_auths setup for:
    • initialize by admin
    • transfer_admin attempted by unauthorized_caller
  • Added direct failure assertion and retained admin-state invariant assertion.

• test_transfer_admin_chain now executes and verifies the second transfer

  • Removed the misleading “new client as caller” pattern.
  • Switched to explicit per-call auth mocking:
    • admin -> new_admin_1
    • new_admin_1 -> new_admin_2
  • Added the missing second transfer_admin call before asserting final admin.

• Test imports updated for explicit auth mocking

  • Added MockAuth, MockAuthInvoke, and IntoVal to support invocation-scoped auth setup.

let result = distributor
    .mock_auths(&[MockAuth {
        address: &unauthorized_caller,
        invoke: &MockAuthInvoke {
            contract: &distributor_id,
            fn_name: "transfer_admin",
            args: (&new_admin,).into_val(&env),
            sub_invokes: &[],
        },
    }])
    .try_transfer_admin(&new_admin);
assert!(result.is_err());
assert_eq!(distributor.get_admin().unwrap(), admin);