feat: add testing framework dependencies

Add pytest and visual testing dependencies to requirements.txt:
- pytest>=7.0 for test framework
- pytest-cov>=4.0 for coverage reports
- pillow>=10.0 for image processing
- imagehash>=4.3 for visual regression testing

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

requirements.txt

@@ -1 +1,5 @@
-pyyaml>=6.0
+pyyaml>=6.0
+pytest>=7.0
+pytest-cov>=4.0
+pillow>=10.0
+imagehash>=4.3