test: add integration test for buy transaction reverting on insuffici…

[Killerjunior]

test: add integration test for buy transaction reverting on insufficient XLM (#551)

Closes #551

Summary

• Added an integration test in creator-keys/tests/buy_insufficient_xlm_revert.rs to verify that buy_key cleanly reverts with ContractError::InsufficientPayment when the attached amount is 1 stroop below the current bonding curve price.
• Verified that key total supply and buyer holdings remain completely unchanged after the transaction reverts (no state mutation).

Testing

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets -- -D warnings
• cargo test --workspace

Checklist

• Linked issue or backlog item (Add integration test for buy transaction reverting when buyer sends insufficient XLM #551)
• Added or updated creator-keys unit/integration tests for every changed contract behavior, including failure paths for new or reachable ContractError variants
• Ran cargo fmt --all -- --check, cargo clippy --workspace --all-targets -- -D warnings, and cargo test --workspace, or explained exactly why a command was not run
• Reviewed persistent storage changes against docs/storage-key-invariants.md; any storage layout change includes a migration/backward-compatibility note
• Confirmed event names, topic order, payload field order, and field meanings remain compatible with docs/contract-event-conventions.md, or documented the breaking change and versioning plan
• Updated docs for any changed public contract interface, read-only method, event schema, storage behavior, fee logic, or deployment workflow
• Scope stays limited to one contract concern and does not include unrelated formatting, lockfile, generated artifact, or dependency changes