[cyipopt] Auto-select limited-memory Hessian for grey-box models

[aryanjsingh]

Summary

This change updates the CyIpopt solver interface to automatically select the 'limited-memory' Hessian approximation when a grey-box model is present that does not provide exact Hessian information. This prevents poor solver performance when users do not manually specify a Hessian approximation method.

Changes

• pyomo/contrib/pynumero/solvers/ipopt.py: In the CyIpopt solver's solve() method, added logic to detect if the model contains an ExternalGreyBoxBlock whose external model does not provide methods for Hessian evaluation. If such a block is found and the user has not explicitly set the 'hessian_approximation' option, it is automatically set to 'limited-memory' to ensure robust solver behavior.

Related Issue

Closes #3891

[blnicho]

There is already an open PR (#3892) to fix issue #3891 so closing this.