fix(safe-math): fix #20 by implementing overflow-safe arithmetic across contracts

[Dopezapha]

Closes #20

PR Description

This PR resolves issue #20 by addressing arithmetic overflow and underflow risks in calculations across the StepFi contracts. Multiple contracts performing math on i128 values for loans, late fees, share calculations, and boosts previously had no explicit overflow checks, which posed a security risk where extremely large numbers could wrap or cause unhandled panics.

I replaced raw operators (+, -, *) on financial values with explicit checked methods inside a standardized safe_math utility module implemented in each contract. When an overflow or underflow occurs, the contract now returns a custom, typed error (e.g., Overflow or Underflow) instead of panicking, which allows client applications and invoking contracts to handle the error properly.

Changes Made

• Added local safe_math.rs utility modules to all 5 core contracts (and the vouching contract) to implement checked addition, subtraction, multiplication, and division on i128, u64, and u32 variables.
• Modified contract lib.rs and errors.rs files for creditline-contract, liquidity-pool-contract, reputation-contract, parameters-contract, vendor-registry-contract, and vouching-contract to integrate the safe math helper functions and custom error variants.
• Added Overflow and Underflow variants to all contract-specific error enums to replace unhandled VM panics with typed error responses.
• Added a test_safe_math_boundaries unit test in contracts/creditline-contract/src/tests.rs verifying that overflow, underflow, and divide-by-zero occurrences on extreme boundary values return the correct typed errors.