Add regression test for buy and sell producing symmetric price impact

[Chucks1093]

Summary
Buying and then selling the same amount of keys should return the price close to the original, accounting for fees. A regression test that verifies the price impact is symmetric confirms the bonding curve behaves as expected in both directions.

Scope

• add a regression test that records the buy price at a given supply
• buy a defined amount and then sell the same amount
• assert the sell proceeds minus fees are within the expected range of the original buy price

Acceptance Criteria

• the test confirms buy and sell price impact is symmetric within the expected tolerance
• the fee deduction is accounted for in the assertion
• the test documents the expected price relationship in a comment

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[Dannyswiss1]

@Dannyswiss1 has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Let me work on this

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Congratulations, @Dannyswiss1! 🎉 Your application was accepted by the repo's maintainers, and the issue is due on June 2, 2026.

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