Feat/borrow interest overflow error

[Abidoyesimze]

Summary

Make interest conversion overflow explicit in calculate_interest:
Change return type to Result<i128, BorrowError>.
Convert with to_i128().ok_or(BorrowError::Overflow)? instead of capping to i128::MAX.
Propagate/handle the new error at call sites (borrow, repay, and get_user_debt).
Add tests to validate overflow behavior and keep non-overflow math checks intact.
Motivation
Previously, an I256→i128 conversion overflow would silently clamp to i128::MAX, masking arithmetic errors and potentially distorting accounting. This change makes overflow explicit, preventing silent state corruption.
What’s Changed
stellar-lend/contracts/lending/src/borrow.rs
calculate_interest(...) -> Result<i128, BorrowError>
Use ok_or(BorrowError::Overflow)? on I256→i128 conversion.
Propagate errors in borrow and repay.
Keep get_user_debt non-fallible by ignoring overflow (preserves method signature).
Tests
lending/src/borrow_test.rs: add test_interest_overflow_returns_error.
lending/src/math_safety_test.rs: adapt to Result, keep a non-overflow assertion path.
Acceptance Criteria
Overflow in interest conversion returns BorrowError::Overflow.
Tests verify overflow is surfaced and callers propagate the error correctly.
No panics on overflow.

closes #16

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