Fix/withdrawal overflow

[AugistineCreates]

Ths pr closes #42

Summary

This PR resolves a critical overflow bug in WithdrawalLimits::validate where the sum already_withdrawn + amount could overflow an i128, potentially causing a panic or incorrect validation.

Changes Made

1. Added checked arithmetic

   • Replaced unchecked addition with checked_add.
   • Returns an explicit anyhow! error if overflow occurs ("Withdrawal arithmetic overflow: already_withdrawn + amount exceeds i128").

2. Updated error handling

   • Adjusted the error message for total‑cap violations to use the safely‑computed new_total.

3. Added overflow test

   • New test overflow_error_when_sum_exceeds_i128 verifies that an overflow triggers an error instead of panicking.
   • Uses WithdrawalLimits::new(1, i128::MAX, None) to allow the large amount and provoke overflow.

4. Documentation

   • Inline comments clarify the purpose of the overflow check.

Impact

• Prevents runtime panics caused by arithmetic overflow.
• Improves reliability of withdrawal limit validation, especially in misconfigured deployments with extremely large caps.

Verification

• Ran cargo test – all existing tests pass, and the new overflow test succeeds.
• The branch fix/withdrawal-overflow is set as the upstream tracking branch and pushed to GitHub.