fix: return already-completed send without re-paying on the node [AMB-2985]

[bufo24]

Summary

Transactions.send() now checks the transaction create_send returns: if its status is already COMPLETED, the SDK short-circuits and resolves the send as a success instead of executing the node payment.

This covers two cases where the backend returns an existing settled transaction instead of creating a new one:

• a genuine duplicate send (same payment hash, already paid)
• an idempotency-key replay

Without this change, the SDK would take that already-settled transaction and try to pay the invoice again on the node, which fails.

Return shape

{
  transaction, // the existing COMPLETED transaction, unchanged
  payment: {
    status: 'SUCCEEDED',
    paymentHash: transaction.payment_hash,
    feeSat: transaction.fee,       // already sats today — see note below
    paymentPreimage: undefined,    // see note below
  },
}

• paymentPreimage is always undefined on this path. Checked packages/core/schema/rails.graphql — PaymentsTransaction has no preimage field (there's an unmerged amboss-nodify branch, feat/AMB-2698-paymentstransaction-preimage-adjacent work, that would add one, but it isn't live). Only a payment actually executed on the node (the existing path below the new check) can return a preimage.
• feeSat maps directly from transaction.fee with no numeric conversion — both are sats. Verified against amboss-nodify's src/modules/lnd/handlers/offchain.ts on main, which writes payments_transaction.fee from LND's safe_fee (sats).
  • This is a contract, not a coincidence. An earlier attempt to switch that column to msat (amboss-nodify#826) was closed unmerged — it would have broken amboss-cron's routing-fee reimbursement math by 1000x. It has since been decided that the SDK will not handle msat: NodePaymentResult.feeSat stays sats-only and will never carry a /1000 conversion, so whatever the ledger stores underneath must still present sats to the SDK. Tracked in AMB-2990.

Sequencing (important)

This PR must merge and be released before the sibling amboss-rails-api PR (fix: return completed transaction instead of throwing on duplicate payment hash [AMB-2985]).

If the rails-api change lands first while consumers are still on an older SDK version, the SDK will receive the completed transaction from create_send and attempt to re-pay it on the node — producing a confusing node-side error where users previously got a clean "already paid" style result. Ship this SDK change and let it propagate to consumers first.

Test plan

• pnpm run typecheck
• pnpm run build
• pnpm run test (36/36 passing, including 2 new cases)
  • COMPLETED transaction from create_send → resolves success without invoking the node
  • non-COMPLETED transaction → still executes the node payment exactly as before (regression check)
• pnpm run format:check
• Sandbox path is untouched (existing test unchanged, still passing)

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