Implement SDK payment error classification

[El-swaggerito]

Description

Difficulty: Expert

Type: Feature

Summary

Implement typed payment error classification for PocketPay SDK payment helpers.

Current Behaviour

Payment failures may surface as raw errors that are hard for applications to display or recover from.

Expected Behaviour

The SDK should classify validation, network, account, asset, fee, and submission failures into stable error categories.

Suggested Implementation

Create error categories, mapping utilities, payment integration, tests, fixtures, and docs.

Files or Areas Likely Affected

• src/payments/
• src/transactions/
• src/errors/
• src/diagnostics/
• tests/
• docs/

Acceptance Criteria

• Payment error categories are implemented.
• Validation, network, account, asset, fee, and submission errors are represented.
• Payment helpers return typed errors.
• Diagnostics avoid sensitive values.
• Tests cover each major error category.
• Documentation explains how apps should handle errors.

Additional Notes

This improves SDK reliability and review quality.