No test for a removed signer using a stale approval reference

[yusuftomilola]

auth_tests.rs doesn't cover a signer who has been removed from the registry attempting to approve a transfer using a stale signer reference.

[rupesh-kumar-sah]

Hi everyone,

I am interested in working on this issue.
Plan:

1. Create reproduction and validation suites for No test for a removed signer using a stale approval reference to ensure regression-free behavior.
2. Ensure zero flake and 100% green pass rate across the test suite.

Ready to deliver a clean, tested PR within 12–24 hours. Please assign!