test(learn-token): add i128 boundary tests for token operations

[lmduy2612]

Description

Fixes #639

This PR adds comprehensive unit test coverage for numeric boundaries across token operations in LearnToken:

• test_approve_with_i128_max_amount: Verifies setting allowance to i128::MAX records exactly and does not overflow.
• test_transfer_of_exact_balance_leaves_zero: Verifies transferring exactly 100% of an account's balance updates sender to 0 and recipient correctly.
• test_burn_of_exact_balance_leaves_zero: Verifies burning 100% of balance leaves account at 0 and decrements total_supply cleanly.
• test_mint_to_i128_max_supply_cap: Verifies minting up to i128::MAX total supply and balance boundary without panics.
• test_allowance_operations_at_i128_max: Verifies decrease_allowance and subsequent increase_allowance operate reliably on maximum values.

Verification

• All newly added tests pass cleanly:
  cargo test --test token_tests test_approve_with_i128_max_amount
cargo test --test token_tests test_transfer_of_exact_balance_leaves_zero
cargo test --test token_tests test_burn_of_exact_balance_leaves_zero
cargo test --test token_tests test_mint_to_i128_max_supply_cap
cargo test --test token_tests test_allowance_operations_at_i128_max
• Code formatted with rustfmt.