test: cover create_vault amount boundaries

[dangelo352]

Summary

• keep randomized success sampling on the valid interior while preserving explicit MIN_AMOUNT/MAX_AMOUNT edge tests
• assert MIN_AMOUNT - 1, MAX_AMOUNT + 1, zero, and negative amounts return Error::InvalidAmount
• document the amount-boundary strategy and funding separation in TESTING_GUIDE.md

Closes #231

Validation

• cargo test --test proptest_amounts -- --nocapture
• cargo test
• rustfmt tests/proptest_amounts.rs
• git diff --check

Note: cargo fmt --check currently reports a pre-existing formatting diff in tests/proptest_timestamps.rs, which this PR does not modify.

[1nonlypiece]

hey, good work, but this can't go in until you're assigned to #231 — that's our process to avoid two people on the same thing. closing for now; claim the issue first and re-raise. https://discord.gg/xvNAvMJf