feat: implement interest accrual and last_update_timestamp

[Mrwicks00]

Pull Request: Implement interest accrual logic and last_update_timestamp

Title: feat: implement interest accrual and last_update_timestamp
Branch: feature/interest-accrual
Base: main

Description

This PR implements the interest accrual logic for Soroban credit lines as specified in issue #8. Utilized debt now grows over time based on the interest_rate_bps and the time elapsed since the last_update_timestamp.

Closes #8

Changes Made

• Data Model: Added last_update_timestamp to CreditLineData.
• Accrual Logic: Implemented compute_accrued_interest helper to calculate interest using overflow-safe arithmetic (i128) and a simple interest model (linear in time).
• Accrual Helper: Implemented update_accrued_interest to dynamically add interest to utilized_amount and update the last_update_timestamp.
• Integration: Integrated the update_accrued_interest helper before relevant storage accesses in:
  • draw_credit
  • repay_credit
  • update_risk_parameters
• Read Operations: Integrated compute_accrued_interest into get_credit_line so queries reflect real-time effective debt.
• Testing:
  • Validated backwards compatibility with all existing contract flows.
  • Added new comprehensive tests specifically simulating linear time passage for interest accrual:
    • test_interest_accrual
    • test_interest_accrual_zero_rate
    • test_interest_accrual_large_time (tests 10-year span of simple interest calculation).
• Maintained minimum 95% test coverage.

Verification

Tests have been run locally using cargo test -p creditra-credit. All tests passed successfully.

[Mrwicks00]

@greatest0fallt1me please check

[greatest0fallt1me]

resolve the conflict

[greatest0fallt1me]

@Mrwicks00

[greatest0fallt1me]

@Mrwicks00 please resolve the conflicts

[greatest0fallt1me]

@Mrwicks00 Can you resolve the conflicts?