[vault-contract] Replace .expect() with custom errors in critical paths

[Villarley]

Based on SECURITY_AUDIT_VAULT_CONTRACT-V1.0.0.md (finding F-04).

Severity: High

Summary

Replace the 14 .expect() calls with ok_or(ContractError::XxxNotFound)? in functions that return Result.

Acceptance criteria

• Add ContractError variants for missing storage keys (e.g. EnabledNotFound, RoiPercentageNotFound, etc.)
• Replace .expect() in claim, availability_for_exchange with proper error propagation
• Document or refactor getters that assume initialized state
• Tests pass
• Scout no longer reports unsafe expect at lines 142, 152, 158, 173

[ryzen-xp]

Hi maintainers 👋
I’m a blockchain developer with strong expertise in Stellar, Rust,TS and Stellar SDK integrations. I’ve worked on multiple Stellar-based smart contract and backend systems.

I understand the scope of this issue and can deliver a reliable, production-quality solution with proper tests and documentation.
Plz assign me !!

ETA: 12 hours

[Idrhas]

Hi Villarley, I'd like to work on this issue, I have experience with the relevant technologies and ecosystem and have previously contributed to similar issues. I’m comfortable working with the codebase and familiar with standard practices.

[CoKeFish]

assign me