feat(testing): implement Phase 4 advanced testing

- Add Hypothesis property-based tests (19 tests)
- Add Syrupy snapshot tests (7 tests)
- Add pytest-benchmark performance tests (11 tests)
- Fix Hypothesis plugin compatibility by creating MockModule class
- Update pytest.ini to exclude .hypothesis directory
- Add .hypothesis/ to .gitignore
- Update requirements-dev.txt with testing dependencies
- Mark Phase 4 complete in backend-testing-improvement-plan.md

All 947 tests passing.

.gitignore

@@ -19,3 +19,6 @@ model_cache/
 vue-widgets/node_modules/
 vue-widgets/.vite/
 vue-widgets/dist/
+
+# Hypothesis test cache
+.hypothesis/