refactor(wangapayfast-rs): add structured error model for API and ITN

[mitch1009]

Summary

Expand error handling to provide clear category-specific failures for API and ITN flows.

Scope

• Add typed error variants for network, auth/signature, validation, payfast business errors, decode errors
• Preserve source chaining and context details
• Improve display/debug output

Acceptance Criteria

• Errors are category-specific and actionable
• Existing call sites compile with updated model
• Tests cover representative error mapping