test(learn-token): add allowance_spender_count unit tests (fixes #635)

[lmduy2612]

Closes #635.

Summary of Changes

Adds comprehensive unit tests for allowance_spender_count in contracts/learn-token:

• Owner with no allowances: verifies returning 0 (test_allowance_spender_count_returns_zero_for_owner_with_no_allowances).
• Single allowance: verifies returning 1 after one approval (test_allowance_spender_count_returns_one_after_one_approve).
• Multiple allowances: verifies returning 2 after approvals to two distinct spenders (test_allowance_spender_count_returns_two_after_two_approves_to_different_spenders).
• Expired allowance persistence: verifies that spender count does not decrease when an allowance expires, proving the spender registry remains intact until explicit cleanup (test_allowance_spender_count_does_not_decrease_when_allowance_expires).

Verification

• cargo test --test token_tests test_allowance_spender_count: 4/4 tests pass.
• Adheres strictly to project coding standards and test conventions.