[vault-contract] Fix overflow in TotalTokensRedeemed

[Villarley]

Based on SECURITY_AUDIT_VAULT_CONTRACT-V1.0.0.md (finding F-05).

Severity: High

Summary

Use checked_add for total_redeemed + token_balance (line 198).

Acceptance criteria

• Use checked_add and propagate ContractError::ArithmeticOverflow on overflow
• Tests pass
• Scout no longer reports overflow at line 198

[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