Add SDK integration fixtures for payment flows

[El-swaggerito]

Description

Difficulty: Expert

Type: Test

Summary

Add deterministic integration fixtures for SDK payment flows.

Current Behaviour

Payment tests may rely on ad-hoc data or incomplete mocks.

Expected Behaviour

The SDK should include reusable fixtures covering account states, payment inputs, transaction previews, errors, and receipts.

Suggested Implementation

Create fixture files, test helpers, documentation, and example usage.

Files or Areas Likely Affected

• tests/
• src/payments/
• src/transactions/
• fixtures/
• docs/

Acceptance Criteria

• Payment integration fixtures are added.
• Valid account and payment states are represented.
• Invalid and failing states are represented.
• Tests consume the fixtures.
• Fixtures avoid real secrets.
• Docs explain fixture usage.

Additional Notes

This improves contributor test quality.