Fuzz: `i128` boundary amount handling

[nanaf6203-bit]

Problem Statement
Existing fuzz tests cap amount at 10^12. The i128 boundaries
(i128::MAX, i128::MIN, 0) are not exercised.

Why it matters
A tip() overflow at any cast or arithmetic step is a fatal DoS.

Technical Context
Existing proptests in src/fuzz.rs.

Expected Outcome
A fuzz that picks amounts drawn from (-1, 0, 1, i128::MAX, i128::MIN, u64::MAX, u64::MAX as i128, ...) and asserts the contract panics with
InvalidAmount or BelowMinimum rather than overflowing.

Acceptance Criteria

• cargo test fuzz runs ≥ 1000 cases for the new range.
• No raw panics without a typed error.

Files/Modules Likely Affected

• src/fuzz.rs

Difficulty Easy. Estimated Effort S.

Labels
area:testing, area:fuzzing,
area:smart-contract-safety, priority:high

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Imported from docs/open-issues-draft.md as draft issue #30 (commit 7becd4b). Original draft-order cross-references in this body have been remapped to current GitHub issue numbers at import time.

[GBOYEE]

Hi team — I'd like to take this on as part of the GrantFox campaign. I'll implement it and open a PR referencing this issue shortly.

[irefavor15-dotcom]

HI MAINTAINER

I would like to request assignment to work on this issue. I am ready to expand the fuzzing suite in src/fuzz.rs to exercise extreme boundary values including i128::MAX, i128::MIN, 0, and u64::MAX, ensuring that all tip and arithmetic operations gracefully fail with typed errors like InvalidAmount or BelowMinimum instead of raw panics, and validating with robust proptest execution. Please assign this ticket to me so I can get started.

[grantfox-oss]

🦊 GrantFox — @irefavor15-dotcom has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #90)
2. Your PR will be reviewed by the StellarTips maintainers

Good luck! Track your progress on GrantFox.