Add slippage protection parameters to buy and sell functions

[Chucks1093]

Summary

Between the moment a user sees a price quote and the moment their transaction lands on chain, other trades can move the price. Without slippage protection the user has no on-chain guarantee about what they will pay or receive. Adding a max_price param to buy and min_proceeds to sell gives users a hard limit that reverts the transaction if the market moved too far before their trade executed.

Scope

• Add optional max_price: Option<i128> parameter to the buy function
  • If provided and the computed buy price exceeds max_price, the transaction panics with a new SlippageExceeded error variant
• Add optional min_proceeds: Option<i128> parameter to the sell function
  • If provided and the computed sell proceeds are below min_proceeds, the transaction panics with SlippageExceeded
• Add SlippageExceeded to the contract error enum
• Quote view functions remain unchanged — they are read-only and do not need the parameter

Acceptance Criteria

• Buy with max_price reverts with SlippageExceeded when price exceeds the limit
• Buy with max_price succeeds when price is at or below the limit
• Sell with min_proceeds reverts with SlippageExceeded when proceeds fall below the limit
• Sell with min_proceeds succeeds when proceeds meet or exceed the limit
• Omitting either parameter (passing None) preserves existing behavior with no revert
• SlippageExceeded is a stable error variant with a fixed discriminant
• Unit tests cover: buy slippage triggered, buy slippage not triggered, sell slippage triggered, sell slippage not triggered, None passthrough

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Coordinate on Telegram

[KingDavid9999]

Hi, I'd like to claim this issue. I'm an active contributor to the Stellar/Soroban ecosystem and have hands-on experience implementing contract-level safety guards in Rust. In my work on AgenticPay, a decentralized freelance payment platform built on Stellar, I've worked extensively with Soroban contract error enums, #[contracterror] discriminant management, and function signature changes that maintain backward compatibility through Option parameters. My approach for this issue: add SlippageExceeded with a fixed discriminant to the existing error enum, extend the buy and sell signatures with Option guards inserted immediately after price/proceeds computation, and cover all five acceptance-criteria test cases using Soroban's test environment to simulate reserve states that trigger and bypass the revert. I'll leave the quote view functions untouched as scoped.

[barry01-hash]

I would love to work on this project

[robertocarlous]

Hello maintainer, i am a backend developer, with strong technical experience, i understand the scope of this project, can i work on this issue

ETA:10HRS

[grantfox-oss]

🦊 GrantFox — @robertocarlous 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 #392)
2. Your PR will be reviewed by the accesslayerorg 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!

@robertocarlous's PR #398 was approved and merged by @Chucks1093.

🏆 @robertocarlous: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (398 total points). Track your full progress on GrantFox.

👏 Great work, @robertocarlous! Keep contributing to accesslayerorg.