Implement invoice auto-currency conversion

[Kingsman-99]

Label: complexity: high
Points: 200

Description

Payers holding XLM or other Stellar assets cannot pay USDC invoices directly. This issue adds an auto_convert: bool flag to pay() — when true, the contract calls the Stellar DEX to swap the payer's asset to the invoice token before crediting the payment.

Technical Context

Involves lib.rs pay(). Add auto_convert: bool parameter. When true, invoke a DEX swap contract (mock interface in tests) to convert payer's source asset to invoice.token before the standard token_client.transfer(). The swapped amount is what gets credited to invoice.funded.

Acceptance Criteria

• pay() accepts auto_convert: bool parameter
• auto_convert = false produces identical behaviour to current implementation
• When auto_convert = true, DEX swap is invoked before crediting payment
• Swapped amount correctly credited to invoice.funded
• Test mocks DEX swap and verifies invoice funded with converted amount
• All existing cargo tests pass
• cargo clippy passes with zero warnings

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[Georgebingi]

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

Hi I'm a full stack developer I'll like to work on this issue because It picks my interest and Ive got experience with similar projects I'll have it solved in 24 hours or less

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[Emelie-Dev]

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[sonoflawal]

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[Tobiloba0]

@Tobiloba0 has applied to work on this issue as part of the Stellar Wave Program's 5th wave.

Hi maintainer I will like to work on this issue.

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[drips-wave]

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

🧑‍💻 @sonoflawal: Please resolve the issue such that the repo's maintainers have enough time to review your contribution before the due date. You'll earn Points for completing the issue on-time, which will make you eligible for a share of the Stellar Wave Program's reward pool.

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[drips-wave]

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

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