Add Regression Testing for Bug Fixes

Goal

Ensure bug fixes include regression coverage that proves the original problem does not return.

Repository Context

This TypeScript application uses Vitest.

payments/
├── src/
├── tests/
├── package.json
└── AGENTS.md

The repository expects bug fixes to include a regression test when the behavior can reasonably be tested.

The regression test should reproduce the previously incorrect behavior and pass after the fix.

Current AGENTS.md

# Project Instructions

## Development

- Use `pnpm`.
- Keep changes focused on the reported problem.

## Testing

- Run existing tests after fixing a bug.

## Validation

Before completing substantial changes, run:

- `pnpm lint`
- `pnpm typecheck`
- `pnpm test`

Your Task

Add testing guidance requiring a regression test for bug fixes when the affected behavior can reasonably be tested.

Keep the existing validation workflow.