Fix interest accrual calculation for loan_manager contract

[Dherah-77]

closes #456

This PR fixes an issue with the interest accrual logic in the loan_manager contract to ensure accurate and consistent calculation of interest over time.
Corrected the interest accrual formula to align with expected financial logic
Fixed scaling/precision handling to avoid incorrect values
Updated relevant functions to ensure proper accumulation of interest
Improved test coverage for interest-related scenarios
Ran cargo test successfully
All existing and updated tests passed
Verified correctness of interest calculations through test cases

[ogazboiz]

hey, contracts CI is failing on cargo fmt. please run this and push:

cd contracts
cargo fmt --all
git add -A
git commit -m "fix: cargo fmt"
git push