Prevent unhandled promise rejections in LN invoice status checks

[Copilot]

The service could terminate on ERR_UNHANDLED_REJECTION when LN invoice polling hit a rejected promise with an undefined reason. This PR hardens invoice-status checking so failures are contained and converted into safe, non-crashing outcomes.

• Invoice wait flow hardening (src/invoicing.js)

  • Reworked check_invoice_is_paid to avoid new Promise(async ...), which can leak unhandled rejections from the async executor.
  • Split timeout and RPC wait into separately managed promises.
  • Wrapped the RPC path with explicit .catch(() => undefined) and used Promise.race(...) to return the first settled result.
  • Ensured timeout cleanup via finally to avoid stale timers.

• Regression coverage (test/ln_flow.test.js)

  • Added a focused test that simulates LN RPC rejection with undefined and asserts:
    • check_invoice_is_paid(...) resolves to undefined
    • no unhandledRejection process event is emitted

const waitinvoicePromise = this.ln_rpc({ method: "waitinvoice", params })
  .then((res) => res.error ? false : true)
  .catch(() => undefined)

return await Promise.race([waitinvoicePromise, timeoutPromise])