feat(loan_manager): add repayment below minimum error handling and up…

[BigJohn-dev]

Closes #8

✅ Requested error handling updates are in place

I updated the contracts so the user-facing cases now return typed errors instead of panicking:

• In lib.rs, repay now returns LoanError::RepaymentBelowMinimum for partial repayments that fall below the configured floor.
• In lib.rs, set_min_repayment_amount now returns LoanError::InvalidAmount for negative input instead of panicking.
• The underflow path in adjust_total_outstanding is still a panic, but it is now documented as an internal invariant that should be unreachable from normal inputs.
• In lib.rs, cooldown enforcement now returns PoolError::WithdrawalCooldownActive instead of panicking.
• The related assertions were updated in test.rs and test.rs.

I also checked editor diagnostics afterward, and there are no reported Rust errors in the touched files.