Add regression test for protocol fee recipient receiving correct amount on sell

[aniokedianne]

Summary
Event tests confirm the sell fee fields are emitted, but there is no regression test that verifies the protocol fee recipient actually receives the correct token amount after a sell. A direct balance assertion closes this gap.

Scope

• add a regression test that records the protocol fee recipient balance before a sell
• perform a sell and assert the recipient balance increased by the expected fee amount
• assert the fee amount matches the protocol fee bps applied to the sell price

Acceptance Criteria

• the protocol fee recipient balance increases by the correct fee amount after a sell
• the fee amount matches the bps calculation
• the test is distinct from existing sell event field tests

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[wheval]

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

Hello i would love to work on this.

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

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This issue has been completed by @wheval as part of the Stellar Wave Program's 5th Wave 🥳

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