Payment Expiry Guard

[Kingsman-99]

Description

Invoices carry an expiresAt timestamp, but the SDK does not enforce this on the transaction side by setting timebounds.maxTime on built transactions. A payment submitted after an invoice expires should be rejected at the ledger level, not merely by SDK-side checks. The expiry guard must translate invoice expiry into transaction timebounds and reject submission attempts where the expiry has already passed.

Technical Context

Extend src/txBuilder.ts to set timeBounds using TransactionBuilder.setTimeout() from @stellar/stellar-sdk. Reads InvoiceRecord.expiresAt from src/types.ts. Add PaymentExpiredError to src/errors.ts. Pre-submission check added to src/preflightChecker.ts. src/deadlineEngine.ts provides the remaining-seconds calculation that feeds into setTimeout().

Acceptance Criteria

• txBuilder.build() automatically calls setTimeout(remainingSeconds) when building a payment for an invoice with expiresAt set
• Throws PaymentExpiredError synchronously when expiresAt is in the past at build time, before any network call
• Respects a configurable expiryBufferSeconds (default 30) subtracted from the remaining time to account for clock skew
• Zero remaining time after buffer causes PaymentExpiredError with a human-readable message including the expired timestamp
• Unit tests cover: expiry 60 s away (builds OK), expiry 20 s away with 30 s buffer (throws), already-expired (throws)
• All CI checks (npm test, npm run build, ESLint/TypeScript) pass and the branch has no merge conflicts

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[aburex12345]

@aburex12345 has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

Hello Maintainer, I’ve thoroughly read the issue description and understand the expected outcome. I’d love to work on it.

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[lekescrew22]

@lekescrew22 has applied to work on this issue as part of the Stellar Wave Program's 7th wave.

i would love to work on this issue

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