Add integration test for buy event emitted with correct fields on successful purchase

[Chucks1093]

Summary

The buy function emits a contract event on success but no integration test verifies the event fields are correct. A test should perform a buy transaction and assert the emitted event contains the expected buyer, creator, quantity, price, and timestamp fields.

Scope

• Deploy the contract and perform a buy of 1 key for a seeded creator
• Extract the contract events from the transaction result meta
• Assert one buy event is emitted with fields: buyer, creator_id, quantity, price_paid, ledger
• Assert field values match the transaction inputs and current ledger number

Acceptance Criteria

• Exactly one buy event emitted per transaction
• All five fields present with correct values
• price_paid matches bonding curve price at the supply step before the buy
• ledger matches the transaction ledger number

ETA: 12 hours

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