Add per-call max_fee_bps slippage guard to deduct

[greatest0fallt1me]

Description

This is a smart-contract issue for the GrantFox campaign. Allow callers to specify a max acceptable fee in basis points; revert if exceeded.

Requirements and Context

• Param max_fee_bps
• Revert with Slippage error
• Default = u16::MAX
• Test slippage path
• Must be secure, tested, and documented
• Should be efficient and easy to review

Suggested Execution

1. Fork the repo and create a branch

   git checkout -b feature/max-fee-bps

2. Implement changes
   • contracts/vault/src/lib.rs
   • contracts/vault/src/errors.rs
3. Test and commit
   • Run the repo's standard test suite and lint
   • Cover edge cases; include output in the PR

Example commit message

feat: max_fee_bps slippage guard on deduct

Acceptance Criteria

• Param accepted
• Revert correct
• Default safe
• Tests cover

Guidelines

• Minimum 95% test coverage with cargo test
• require_auth on every state-changing entrypoint
• Overflow-safe math; no unwrap() in production paths
• Clear NatSpec-style /// rustdoc
• Timeframe: 96 hours

[Okorie2000-code]

@Okorie2000-code has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi Maintainer, I'd love to work on this issue. I understand the requirements and will submit a clean, well-tested PR.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @Okorie2000-code to this issue.

[drips-wave]

Congratulations, @Okorie2000-code! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 30, 2026.

🧑‍💻 @Okorie2000-code: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

Warning

When opening a PR, please link it to this issue to ensure it gets tracked accurately. Points are awarded when this issue is marked as completed by the maintainer.

🤠 Repo maintainers: Please keep an eye on the contributor's progress and review their work before the due date. You can manage this issue, including adjusting its complexity and points, here.

🌊 Happy Wave 🌊

[drips-wave]

This issue has been completed by @Okorie2000-code as part of the Stellar Wave Program's 6th Wave 🥳

😎 @Okorie2000-code: You earned 100 Points for completing this issue! After the current Wave ends, you'll be eligible for a percentage of the Wave's reward pool based on the percentage of total points you've earned. Learn more here. You can also Leave a review to share your experience working on this issue.

🧑‍💻 Repo maintainers: How'd the contributor do? Leave a review to share your experience working with them.

[grantfox-oss]

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

@Okorie2000-code's PR #519 was approved and merged by @greatest0fallt1me.

🏆 @Okorie2000-code: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (153 total points). Track your full progress on GrantFox.

👏 Great work, @Okorie2000-code! Keep contributing to CalloraOrg.