Add swap-token preview for pay_with_token

[Kingsman-99]

Labels: complexity: high

Implement previewTokenSwap(invoiceId, sourceToken, sourceAmount) that simulates what the DEX would return before calling pay_with_token. Uses the configured DEX contract's simulation interface.

Acceptance criteria:

• Returns { estimatedOutput: bigint, priceImpactBps: number, route: string[] }
• Calls simulate_transaction on the DEX contract rather than submitting
• Returns error with message if no DEX is configured on the invoice
• Exported with full TypeScript types

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[midexol]

@midexol has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hii maintainer, I'd really like to work on this issue, please assign

ℹ️ Repo Maintainers: To accept this application, review their application or assign @midexol to this issue.

[meem08]

@meem08 has applied to work on this issue as part of the Stellar Wave Program's 6th wave.

Hi
Please assign me this task. I’ll implement previewTokenSwap using the DEX simulation interface, return fully typed swap preview data, handle missing DEX configuration with clear errors, and export the new API.

ℹ️ Repo Maintainers: To accept this application, review their application or assign @meem08 to this issue.