test: add unit tests for payment processors

[phoenix-server]

Summary

The following files have near-zero unit test coverage (~16% statements). Each payment processor should be tested in isolation using stubs/mocks for HTTP calls.

Files to cover

• src/payments-processors/lnbits-payment-processor.ts (12%)
• src/payments-processors/lnurl-payments-processor.ts (23%)
• src/payments-processors/nodeless-payments-processor.ts (14%)
• src/payments-processors/null-payments-processor.ts (40%)
• src/payments-processors/opennode-payments-processor.ts (17%)
• src/payments-processors/zebedee-payments-processor.ts (17%)

Out of scope

• src/services/payments-service.ts — covered by test: add unit tests for MaintenanceWorker and PaymentsService #440
• Payment callback controllers — see separate issue

Acceptance criteria

• Each processor has a corresponding .spec.ts under test/unit/payments-processors/
• Happy path and error cases covered for each processor's public methods
• No real HTTP calls — use sinon stubs
• npm run cover:unit passes without regressions

[snakefood3232]

The unit test coverage for these six payment processors is currently too low. I'd handle this by writing isolated test suites with sinon stubs for all HTTP dependencies, covering both success and error paths. I can have a PR up within a day, as I've built similar test suites for payment systems in TypeScript before.

[phoenix-server]

Thanks @snakefood3232! Sounds like a great fit — go ahead and open a PR when you're ready and we'll review it.