fix(payments): surface payment preimage on send result [AMB-2789]

[wthrajat]

What this does

The send functions dropped the payment preimage that LND and litd already return in the send-payment stream. This adds it.

Why

The preimage is the proof that a payment went through. It's needed for L402 and any proof-of-payment flow. Before this, the only way to get it was setting AMBOSS_SDK_DEBUG=1 and reading the raw stream by hand. An external builder flagged this as their main blocker.

Changes

• NodePaymentResult now has an optional preimage field.
• The LND and litd stream update types now include payment_preimage.
• sendLndPayment and sendAssetPayment copy it into the terminal result.
• Updated the lnd test to assert the preimage is surfaced.

No behavior change for existing callers — it's a new optional field.

Linear: AMB-2789

[wthrajat]

Yes, expected. These send functions are node-direct — they call the customer's own LND/litd REST with the decrypted admin macaroon (self-custody), so the preimage is read straight from the node's payment stream. The lnd path does the same; it's symmetric.

Our GraphQL API does expose transaction.preimage, but only once a transaction is COMPLETED (it's pulled from the details Nodify writes on settlement). For self-custody sends the backend doesn't observe settlement yet, so that field stays null — that's the separate PENDING-send gap (AMB-2790). When we build that path, the preimage the client reports should get persisted and surface on the API too.

So the node stream is the only place a self-custody send's preimage exists today; API parity is tracked in AMB-2790.