[ogazboiz]

Summary

Adds a consolidated integration test binary (creator-keys/tests/buy_sell_revert_paths.rs) covering the two most critical trading failure paths — buying with insufficient XLM and selling more keys than a holder owns — behind a single shared setup helper, so the revert-path assertion pattern stays consistent across buy and sell.

• Insufficient buy: invokes buy_key with an attached amount one stroop below the current price; asserts the call reverts with ContractError::InsufficientPayment and that key supply and buyer holdings are unchanged.
• Oversell: seeds a holder with exactly two keys via buy transactions, then attempts to sell three. sell_key settles one key per call (there is no quantity argument), so the three-key sell is issued as three sequential sells; the first two drain the position and the third — the oversell — is asserted to revert with ContractError::InsufficientBalance while leaving holder balance and creator supply unchanged.
• Both test cases use a shared setup() helper that deploys the contract and seeds a creator.

Implementation note

The contract's sell_key entrypoint settles a single key per call, so the "sell quantity 3" scenario is modeled as three sequential single-key sells where the third exceeds the holder's holdings. This keeps the test faithful to the actual contract API; no production logic was changed.

Test plan

• cargo test -p creator-keys --test buy_sell_revert_paths — 2 passed, 0 failed
• Buy reverts with InsufficientPayment when the payment is 1 stroop short
• Supply and buyer holdings unchanged after the failed buy
• Sell reverts with InsufficientBalance when the sell exceeds holdings
• Holder balance and creator supply unchanged after the failed oversell
• Shared setup helper used by both test cases
• cargo fmt --all -- --check clean
• cargo clippy --test buy_sell_revert_paths -- -D warnings clean

Closes #571

[Chucks1093]

I see what you have done here, the buy and sell failure paths are properly tested so edge cases like insufficient funds and out-of-stock are caught early. Nice one.