docs(tests): enhance TEST_STYLE_GUIDE with async patterns and best practices

docs(tests): enhance TEST_STYLE_GUIDE with async patterns and best practices

- Add Testing Philosophy section with core principles
- Add Async Testing Patterns section (async/await, Promise rejection, timeout)
- Add Error & Exception Testing section with examples
- Soften AAA comments rule (required for complex, optional for simple tests)
- Add anti-pattern #6: Non-strict assertions guidance
- Align with Node.js test runner best practices 2024-2025