ci: add CI test pipeline and refactor tests to pytest format

- Add .github/workflows/ci.yml: runs pytest on Python 3.10-3.13 across
  ubuntu/macos/windows on push and PR to main
- Add [project.optional-dependencies] test group and [tool.pytest.ini_options]
  to pyproject.toml
- Replace all 11 _test_*.py plain scripts with proper test_*.py pytest files:
  each test file uses test functions with assertions instead of print statements,
  covering SVM, SVR, CQR, ElasticNet, FairSVM, MF, monotonic, multiclass,
  path solution, sklearn mixin, and warm-start
- 40 tests pass locally

Author: statmlben

.github/workflows/ci.yml

@@ -0,0 +1,36 @@
+name: CI Tests
+
+on:
+  push:
+    branches: [main]
+    paths-ignore:
+      - "**.md"
+      - "doc/**"
+      - "LICENSE"
+      - ".gitignore"
+  pull_request:
+    branches: [main]
+
+jobs:
+  test:
+    name: "Python ${{ matrix.python-version }} / ${{ matrix.os }}"
+    runs-on: ${{ matrix.os }}
+    strategy:
+      fail-fast: false
+      matrix:
+        os: [ubuntu-latest, macos-latest, windows-latest]
+        python-version: ["3.10", "3.11", "3.12", "3.13"]
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - name: Set up Python ${{ matrix.python-version }}
+        uses: actions/setup-python@v5
+        with:
+          python-version: ${{ matrix.python-version }}
+
+      - name: Install package and test dependencies
+        run: pip install ".[test]"
+
+      - name: Run tests
+        run: pytest -v --tb=short

pyproject.toml

@@ -30,6 +30,15 @@ py-modules = ["build"]
 # Only build on CPython
 build = "cp*"
 
+[project.optional-dependencies]
+test = ["pytest >= 7.0"]
+
+[tool.pytest.ini_options]
+testpaths = ["tests"]
+python_files = ["test_*.py"]
+python_classes = ["Test*"]
+python_functions = ["test_*"]
+
 [build-system]
 requires = ["requests ~= 2.31.0", "pybind11 ~= 3.0.0", "setuptools >= 80.0.0", "wheel >= 0.42.0"]
 build-backend = "setuptools.build_meta"