Add Missing Testing Guidance

Goal

Add testing guidance so agents know when repository tests should be updated and run.

Repository Context

This TypeScript service uses Vitest.

orders-service/
├── src/
│   ├── routes/
│   ├── services/
│   └── utils/
├── tests/
├── package.json
└── AGENTS.md

Available commands:

pnpm lint
pnpm typecheck
pnpm test

The repository expects behavior changes to include relevant test updates.

Current AGENTS.md

# Project Instructions

## Development

- Use `pnpm`.
- Keep HTTP handling in `src/routes`.
- Keep business logic in `src/services`.

## Validation

Before completing substantial changes, run:

- `pnpm lint`
- `pnpm typecheck`

Your Task

Add instructions requiring relevant tests to be added or updated when behavior changes.

Also include the repository's test command in the validation workflow.