Add MAX_AMOUNT and MIN_AMOUNT boundary tests for create_vault in tests/proptest_amounts.rs

[1nonlypiece]

📌 Description

create_vault enforces MIN_AMOUNT ≤ amount ≤ MAX_AMOUNT. Snapshots like
test_amount_below_minimum, test_amount_above_maximum, and
test_amount_exactly_max_allowed exist, but tests/proptest_amounts.rs should
densely cover both exact boundaries and the just-inside/just-outside neighbours,
all returning Error::InvalidAmount on rejection.

Goal: extend the amount property tests so MIN_AMOUNT and MAX_AMOUNT are
accepted, MIN_AMOUNT - 1 and MAX_AMOUNT + 1 are rejected, with randomized
sampling across the valid interior.

🎯 Requirements and Context

• Must be deterministic under a fixed proptest seed.
• Assert exact boundary acceptance and neighbour rejection with Error::InvalidAmount.
• Mint sufficient USDC so rejections come from the amount check, not a balance
  failure (distinguish InvalidAmount from "balance is not sufficient").
• Cover zero and negative amounts explicitly.

🛠️ Suggested Execution

1. Fork the repo and create a branch

git checkout -b feature/amount-boundary-proptests

2. Implement changes

• Tests: extend tests/proptest_amounts.rs with boundary strategies and explicit
  edge vectors at MIN_AMOUNT, MAX_AMOUNT, and their neighbours.
• Docs: add an "Amount Bounds" section to TESTING_GUIDE.md.

3. Test and commit

• Run cargo test and regenerate coverage.
• Cover edge cases: MIN_AMOUNT, MIN_AMOUNT - 1, MAX_AMOUNT, MAX_AMOUNT + 1,
  0, negative.

Example commit message

test: amount boundary property coverage for create_vault

✅ Guidelines

• Minimum 95% test coverage on the amount-validation branches.
• Clear documentation of the boundary cases.
• All tests deterministic under a fixed seed.
• Timeframe: 96 hours.

🏷️ Labels

type-testing · area-contracts · MAYBE REWARDED · GRANTFOX OSS ·
OFFICIAL CAMPAIGN

💬 Community & Support

• Join the Disciplr contributor Discord to coordinate, ask questions, and get
  unblocked fast: https://discord.gg/xvNAvMJf
• Please introduce yourself before you start so we can avoid duplicate work,
  pair you with a reviewer, and merge your PR quickly.

[dangelo352]

I’ll take this one. I’ll add deterministic boundary/property coverage for MIN_AMOUNT/MAX_AMOUNT and the just-outside invalid cases, plus the testing guide note.

[leocagli]

I'd like to work on this issue, please assign me so I can begin implementation.

[hakymulla]

I'd like to apply for this issue. I have open source experience on the stellar ecosystem.