Improve error messages for misconfigured backends

[TheCryptoDonkey]

When a Lightning backend is misconfigured (wrong URL, missing credentials, unreachable host), the first error a developer sees comes from a failed HTTP call deep in the backend code. Adding eager validation in the backend factory functions would surface clear, actionable messages at startup rather than at first request time.

Scope

• Add URL format validation in phoenixdBackend(), lndBackend(), clnBackend(), and lnbitsBackend() factory functions
• Validate that required credentials are present and non-empty (e.g. password for Phoenixd, macaroon for LND, rune for CLN)
• Throw descriptive errors like "phoenixdBackend: password is required" or "lndBackend: url must start with https://"
• Add unit tests for each validation case

Helpful context

• src/backends/phoenixd.ts, src/backends/lnd.ts, src/backends/cln.ts, src/backends/lnbits.ts - the four backend factories
• src/booth.ts lines 92-95 - existing validation pattern in the Booth constructor
• src/backends/conformance.ts - shared test factory (can be extended)
• Estimated effort: 2-3 hours

[shogun444]

Hi, I’d like to take this up.

I can add validation in the backend factory functions to catch misconfiguration early, including URL format checks and required credential validation for each backend. I’ll make the error messages clear and consistent so issues are easy to diagnose at startup.

I’ll also add unit tests covering the validation cases to ensure everything behaves as expected.

ETA: I can have a PR ready within 2–3 hours.