Fix Van der Pol Jacobian calculation

The partial derivative ∂f₂/∂x was incorrect. For the Van der Pol equation:
  f₂ = μ * ((1 - x²) * y - x)

The correct partial derivative is:
  ∂f₂/∂x = μ * (-2xy - 1)

The previous implementation was missing the y term in the derivative.

This fixes the failing test in OrdinaryDiffEq.jl CI.

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

Co-Authored-By: Claude <[email protected]>

Author: ChrisRackauckas

lib/ODEProblemLibrary/src/ode_simple_nonlinear_prob.jl

@@ -63,7 +63,7 @@ function vanderpol_jac(J, u, p, t)
     y = u[2]
     μ = p[1]
     J[1,1] = 0
-    J[2,1] = -2μ*x - 1
+    J[2,1] = μ * (-2*x*y - 1)
     J[1,2] = 1
     J[2,2] = μ * (1 - x^2)
 end