Fix M_operator type error and add CI tests

- Pass w[0] instead of w array to M_operator and M_dense (fixes TypeError)
- Add test job to CI that runs before PyPI upload

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

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: SteveDiamond

.github/workflows/build.yml

@@ -54,8 +54,33 @@ jobs:
           path: dist/*.tar.gz
           compression-level: 0
 
+  test:
+    name: Test wheels
+    needs: [build_wheels]
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: actions/setup-python@v5
+        with:
+          python-version: '3.12'
+
+      - uses: actions/download-artifact@v4
+        with:
+          pattern: cibw-wheels-ubuntu-*
+          path: dist
+          merge-multiple: true
+
+      - name: Install wheel and test dependencies
+        run: |
+          pip install dist/*.whl
+          pip install "diffcp[test]"
+
+      - name: Run tests
+        run: pytest tests/ -v
+
   upload_pypi:
-    needs: [build_wheels, build_sdist]
+    needs: [build_wheels, build_sdist, test]
     runs-on: ubuntu-latest
     environment: pypi
     permissions:

diffcp/cone_program.py

@@ -626,13 +626,13 @@ def solve_and_derivative_internal(A, b, c, cone_dict, solve_method=None,
         D_proj_dual_cone = _diffcp.dprojection(v, cones_parsed, True)
         if mode == "dense":
             Q_dense = Q.todense()
-            M = _diffcp.M_dense(Q_dense, cones_parsed, u, v, w)
+            M = _diffcp.M_dense(Q_dense, cones_parsed, u, v, w[0])
             MT = M.T
         elif mode in ("lsqr", "lsmr"):
             # Convert to csc_matrix for compatibility with C++ bindings
             # (scipy.sparse.bmat may return sparse array in scipy 1.14+)
             Q_csc = sparse.csc_matrix(Q)
-            M = _diffcp.M_operator(Q_csc, cones_parsed, u, v, w)
+            M = _diffcp.M_operator(Q_csc, cones_parsed, u, v, w[0])
             MT = M.transpose()
 
         pi_z = pi(z, cones)