Add PocketPay SDK test-first contribution guide

[El-swaggerito]

Description

Difficulty: Expert

Type: Documentation

Summary

Create a test-first contribution guide for PocketPay SDK.

Current Behaviour

Contributors may update wallet helpers, payment builders, transaction utilities, vault helpers, config, and SDK errors without adding meaningful tests.

Expected Behaviour

The repo should explain when tests are required, what type of tests are expected, and how contributors should prove the behaviour works.

Suggested Implementation

Add documentation with testing examples, no-test justification rules, negative-path expectations, and local commands.

Files or Areas Likely Affected

• src/
• tests/
• docs/
• README.md
• .github/
• package.json

Acceptance Criteria

• Test-first contribution guide is added.
• Testing expectations for wallet helpers, payment builders, transaction utilities, vault helpers, config, and SDK errors are documented.
• Happy-path and negative-path test expectations are included.
• No-test justification rules are documented.
• Local test commands are included.
• README links to the guide.

Additional Notes

This addresses under-tested merged PRs.

[Gipzy8]

Hi, I'd like to apply to this issue.

I have experience writing technical documentation and contribution guides for SDKs, plus hands-on testing experience in TypeScript/JavaScript projects. I understand the problem: changes to wallet helpers, payment builders, transaction utilities, vault helpers, config, and SDK errors can currently be merged without proper test coverage.

My plan: add a test-first guide in docs/ covering when tests are required per module, with happy-path/negative-path examples, no-test justification rules, and local test commands — then link it from the README. Happy to share a first draft for review and iterate on feedback.

[grantfox-oss]

🦊 GrantFox — @Gipzy8 has been assigned to this issue as part of the Official Campaign | FWC26 campaign!

Next steps:

1. Open a Pull Request referencing this issue (e.g., Closes #425)
2. Your PR will be reviewed by the Axionvera maintainers

Good luck! Track your progress on GrantFox.

[grantfox-oss]

🎉 This issue has been marked as completed on GrantFox as part of the Official Campaign | FWC26 campaign!

@Gipzy8's PR #453 was approved and merged by @El-swaggerito.

🏆 @Gipzy8: You earned 35 FoxPoints for this contribution! Your current tier: Explorer (80 total points). Track your full progress on GrantFox.

👏 Great work, @Gipzy8! Keep contributing to Axionvera.