feat: Add test scripts and runner for TheChart application

- Created demo_failing_test.py to demonstrate pre-commit blocking with a failing test.
- Added run_tests.py for executing all tests with coverage reporting.
- Introduced test.py as a quick test runner for the application, providing coverage reports and user-friendly output.

.pre-commit-config.yaml

@@ -65,3 +65,23 @@ repos:
       # - id: uv-export
       # - id: pip-compile
       #   args: [requirements.in, -o, requirements.txt]
+  ########################################################
+  # Run core tests before commit to ensure basic functionality
+  - repo: local
+    hooks:
+      - id: pytest-check
+        name: pytest-check (core tests)
+        entry: uv run pytest
+        language: system
+        pass_filenames: false
+        always_run: true
+        args:
+          [
+            --tb=short,
+            --quiet,
+            --no-cov,
+            "tests/test_data_manager.py::TestDataManager::test_init",
+            "tests/test_data_manager.py::TestDataManager::test_initialize_csv_creates_file_with_headers",
+            "tests/test_data_manager.py::TestDataManager::test_load_data_with_valid_data",
+          ]
+        stages: [pre-commit]