test: add callback controller specs

[Anshumancanrock]

Description

Added unit tests for all payment callback controllers under test/unit/controllers/callbacks/.

Covered:

• valid and invalid payloads per provider
• authentication/signature verification branches
• processor/authorization guard branches
• invoice transition and error paths with dependencies stubbed

Related Issue

Closes #490

Motivation and Context

Callback controllers had low coverage and untested conditional logic. This PR improves confidence in callback handling and reduces regression risk.

How Has This Been Tested?

• npm run test:unit -- 'test/unit/controllers/callbacks/**/*.spec.ts'
• npm run cover:unit

Result:

• 583 passing

Screenshots (if appropriate):

N/A

Types of changes

• Non-functional change (docs, style, minor refactor)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my code changes.
• All new and existing tests passed.

[Copilot]

Pull request overview

Adds unit test coverage for the payment callback controllers to exercise previously untested validation/auth and invoice-transition branches, improving confidence and reducing regression risk in callback handling.

Changes:

• Added unit specs for LNbits, Nodeless, OpenNode, and Zebedee callback controllers.
• Covered malformed payload handling, auth/signature/guard branches, invoice state transitions, and error propagation paths.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
test/unit/controllers/callbacks/lnbits-callback-controller.spec.ts	Adds LNbits callback controller unit tests (HMAC query auth, body validation, invoice transitions, error paths).
test/unit/controllers/callbacks/nodeless-callback-controller.spec.ts	Adds Nodeless callback controller unit tests (signature verification, processor guard, transitions, error paths).
test/unit/controllers/callbacks/opennode-callback-controller.spec.ts	Adds OpenNode callback controller unit tests (payload validation, transitions, error paths).
test/unit/controllers/callbacks/zebedee-callback-controller.spec.ts	Adds Zebedee callback controller unit tests (IP whitelist + processor guard, payload validation, transitions, error paths).

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[cameri]

let's use Sinon fake timers if we are using Date.now() to avoid creating flaky tests

[Anshumancanrock]

@cameri Done! Added Sinon fake timers and a fixed expiry date to prevent test flakiness.