[Contracts] LoanManager uses raw panics for user-facing failures instead of typed LoanError returns

[grantfox-oss]

Telegram (ask questions / claim the issue here first): https://t.me/+DOylgFv1jyJlNzM0

Why this matters

Several user-reachable paths in loan_manager/src/lib.rs panic with strings rather than returning LoanError: repay does panic!("repayment amount below minimum"), set_min_repayment_amount panics on negative input, and adjust_total_outstanding does panic!("total outstanding underflow"). Panics produce untyped traps that clients cannot match on, unlike the existing structured LoanError enum used elsewhere.

Acceptance criteria

• Replace user-reachable panics in repay and set_min_repayment_amount with typed LoanError variants (adding a variant such as RepaymentBelowMinimum if needed)
• Keep internal invariant panics (truly-unreachable underflow) but document why they cannot be triggered by inputs
• Add tests asserting the typed errors are returned for below-minimum repayment and negative min repayment
• Audit the lending_pool withdrawal_cooldown panic for the same treatment

Files to touch

• loan_manager/src/lib.rs

Out of scope

• Reworking the repayment split algorithm
• Changing min-repayment economics

[BigJohn-dev]

I’d like to work on this issue because it aligns closely with the kind of engineering problems I enjoy solving — building reliable, maintainable, and user-focused software. I’m particularly interested in contributing to real-world systems where code quality, scalability, and thoughtful implementation matter.

[Dopezapha]

Hey maintainer, I would love to work on this issue. Kindly assign me the issue. Thanks

[grantfox-oss]

🦊 GrantFox — @BigJohn-dev has been assigned to this issue as part of the Official Campaign campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #8)
2. Your PR will be reviewed by the LabsCrypt maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign campaign!

@BigJohn-dev's PR #37 was approved and merged by @ogazboiz.

🏆 @BigJohn-dev: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (83 total points). Track your full progress on GrantFox.

👏 Great work, @BigJohn-dev! Keep contributing to LabsCrypt.