test: add red contracts for quant core boundaries

tests/test_risk.py

@@ -0,0 +1,54 @@
+"""Risk contribution contracts and validation tests."""
+
+from __future__ import annotations
+
+import numpy as np
+import pytest
+
+from quant_engine.risk import component_var, marginal_risk_contribution, risk_contribution
+
+
+def test_risk_contribution_sums_to_one_for_positive_portfolio_variance() -> None:
+    weights = np.array([0.5, 0.5])
+    covariance = np.diag([1.0, 4.0])
+
+    result = risk_contribution(weights, covariance)
+
+    np.testing.assert_allclose(result, [0.2, 0.8])
+    assert result.sum() == pytest.approx(1.0)
+
+
+def test_zero_variance_portfolio_falls_back_to_equal_contribution() -> None:
+    result = risk_contribution(np.array([0.2, 0.3, 0.5]), np.zeros((3, 3)))
+
+    np.testing.assert_allclose(result, np.full(3, 1 / 3))
+
+
+def test_marginal_and_component_risk_follow_matrix_identities() -> None:
+    weights = np.array([0.25, 0.75])
+    covariance = np.array([[0.04, 0.01], [0.01, 0.09]])
+
+    marginal = marginal_risk_contribution(weights, covariance)
+    component = component_var(weights, covariance)
+
+    np.testing.assert_allclose(marginal, covariance @ weights)
+    np.testing.assert_allclose(component, weights * marginal)
+    assert component.sum() == pytest.approx(weights @ covariance @ weights)
+
+
+@pytest.mark.parametrize(
+    "function",
+    [risk_contribution, marginal_risk_contribution, component_var],
+)
+def test_risk_functions_reject_covariance_shape_mismatch(function) -> None:
+    with pytest.raises(ValueError, match="does not match weights length"):
+        function(np.array([0.5, 0.5]), np.eye(3))
+
+
+@pytest.mark.parametrize(
+    "function",
+    [risk_contribution, marginal_risk_contribution, component_var],
+)
+def test_risk_functions_reject_empty_portfolio(function) -> None:
+    with pytest.raises(ValueError, match="at least one asset"):
+        function(np.array([]), np.empty((0, 0)))