Add unit tests for the buy function emitting a KeyPurchased event with the correct new supply after each purchase

[Chucks1093]

Summary

The KeyPurchased event emitted on each buy should reflect the supply after the purchase, not before. Unit tests should confirm the new_supply field in the event matches the on-chain supply after the transaction and increments correctly across sequential buys.

Scope

• Test a single buy from supply 0 emits KeyPurchased with new_supply: 1
• Test buying 5 keys from supply 10 emits KeyPurchased with new_supply: 15
• Test two sequential buys emit two events with new_supply incrementing correctly
• Assert new_supply in the event equals the supply returned by get_total_supply after the call
• Assert no event emitted on a failed buy

Acceptance Criteria

• new_supply in event equals post-buy on-chain supply
• Sequential buys emit events with correctly incrementing new_supply
• No event emitted on failed buy
• Event new_supply matches get_total_supply return value

ETA: 12 hours

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Coordinate on Telegram

[ezzy156]

Hi @maintainer, I would love to work on this issue
Kindly assign
.