Add LBB relaxed NLP node solver

[bernalde]

Summary

• Add an explicit relaxed_nlp_solver / relaxed_nlp_solver_args role for GDPopt LBB transformed node subproblems that have no unfixed discrete variables.
• Keep the existing MINLP/local-MINLP default when relaxed_nlp_solver is unset, preserving LBB's current global-bounding behavior for nonconvex nodes.
• Reuse GDPopt's existing unfixed-discrete-variable detector for LBB node dispatch and apply the same dispatch to optional local node screening.
• Tighten MindtPy's no-discrete short-circuit to mean no unfixed discrete variables and add a GOA regression for fixed-discrete NLP routing.

Coordination

• This PR does not supersede Fix GDPopt LBB time-limit results #3942. Fix GDPopt LBB time-limit results #3942 fixes the separate LBB time-limit result bug for GDPopt LBB time-limit path calls stale _get_final_results_object #3941.
• Recommended merge order: merge Fix GDPopt LBB time-limit results #3942 first, then refresh/rebase this PR. The runtime changes are independent, but both PRs add imports/tests near the top of pyomo/contrib/gdpopt/tests/test_LBB.py, so this PR may need a small conflict resolution after Fix GDPopt LBB time-limit results #3942 lands.

Tests

• pytest -q pyomo/contrib/gdpopt/tests/test_LBB.py::TestGDPoptLBBNodeSolverDispatch pyomo/contrib/mindtpy/tests/test_mindtpy_no_discrete.py::TestMindtPyShortCircuitRouting::test_goa_short_circuit_fixed_discrete_nlp_uses_nlp_solver
  • Result: 5 passed in 0.99s
• pytest -q pyomo/contrib/gdpopt/tests/test_LBB.py pyomo/contrib/mindtpy/tests/test_mindtpy_no_discrete.py
  • Result: 35 passed, 9 skipped, 2 deselected in 1.70s
• pytest -q pyomo/contrib/gdpopt/tests
  • Result: 78 passed, 35 skipped, 5 deselected in 46.45s
• python -m black --check pyomo/contrib/gdpopt/branch_and_bound.py pyomo/contrib/gdpopt/tests/test_LBB.py pyomo/contrib/mindtpy/algorithm_base_class.py pyomo/contrib/mindtpy/tests/test_mindtpy_no_discrete.py
  • Result: 4 files would be left unchanged

Notes

• The full repository test suite was not run locally. The broader GDPopt test directory and the relevant MindtPy no-discrete tests were run.
• Black emitted its Python 3.13/Python 3.14 parser safety-check warning but exited successfully.

Closes #3945

[bernalde]

Coordination note for #3942:

#3942 and this PR fix different bugs. #3942 fixes the LBB time-limit result path for #3941; this PR fixes continuous-node solver dispatch for #3945. This PR does not supersede #3942.

I checked the combined patches with git merge-tree. The runtime changes are independent, but both PRs add imports/tests near the top of pyomo/contrib/gdpopt/tests/test_LBB.py, so this PR will likely need a small refresh/rebase if #3942 lands first.

Recommendation: merge #3942 first, then refresh this branch to resolve the test_LBB.py import/test insertion conflict.