test(learn-token): add unit tests for increase_allowance overflow and boundaries (#501)

[lmduy2612]

Summary of Changes

Closes #501.

This PR adds comprehensive unit test coverage for increase_allowance in contracts/learn-token, explicitly asserting overflow behavior and edge-case boundary conditions:

1. test_increase_allowance_overflow_panics:

   • Tests increasing allowance when current allowance is near i128::MAX such that current_amount + amount > i128::MAX.
   • Asserts panic with "allowance overflow".

2. test_increase_allowance_at_exact_i128_max_succeeds:

   • Tests increasing allowance precisely up to i128::MAX (e.g. i128::MAX - 50 + 50 == i128::MAX).
   • Verifies allowance equals i128::MAX with no panic.

3. test_increase_allowance_from_i128_max_panics_on_any_positive:

   • Tests attempting to add any positive amount (+1) when allowance is already at i128::MAX.
   • Asserts panic with "allowance overflow".

4. test_increase_allowance_rejects_negative_amount:

   • Asserts panic with "negative amount" when negative value is supplied.

5. test_increase_allowance_rejects_expired_ledger:

   • Asserts panic with "expiration_ledger must be in the future" when expiration ledger is not strictly greater than current ledger.

Verification

• Run cargo test --test token_tests: All 83 unit tests passing.
• Clippy: 0 warnings in token_tests.rs.