Add LBB relaxed NLP node solver

[bernalde]

Fixes #3945.

Summary/Motivation:

GDPopt LBB routes every transformed node subproblem through minlp_solver (or local_minlp_solver for optional local screening), even at nodes where every discrete variable has already been fixed by branching. Those nodes are continuous NLPs, so sending them to a MINLP solver is unnecessary and forces users to have a MINLP solver configured for work a continuous NLP solver could do.

This adds an explicit, opt-in solver role for that case rather than changing the default dispatch, so existing LBB configurations keep their current global-bounding behavior.

This is a replacement for #3946, which GitHub would not allow me to reopen after it was closed. The prior discussion and history remain available there.

Changes proposed in this PR:

• Declare relaxed_nlp_solver and relaxed_nlp_solver_args on GDP_LBB_Solver.CONFIG in pyomo/contrib/gdpopt/branch_and_bound.py, for transformed node subproblems with no unfixed discrete variables.
• Add _get_rnGDP_subproblem_solver(), which reuses GDPopt's existing detect_unfixed_discrete_vars() helper to choose between the relaxed NLP solver and the mixed-integer node solver, and logs which one it picked.
• Preserve current behavior when relaxed_nlp_solver is unset: such nodes continue to go to minlp_solver / local_minlp_solver.
• Apply the same dispatch to the optional local node screening path.
• Factor the existing GAMS reslim time-limit handling into _apply_rnGDP_subproblem_time_limit() so it applies to whichever subsolver is dispatched.
• Tighten MindtPy's continuous short-circuit in pyomo/contrib/mindtpy/algorithm_base_class.py to test for no unfixed discrete variables rather than an empty discrete variable list, and update the surrounding docstring.
• Add TestGDPoptLBBNodeSolverDispatch in pyomo/contrib/gdpopt/tests/test_LBB.py and a MindtPy GOA fixed-discrete routing regression in pyomo/contrib/mindtpy/tests/test_mindtpy_no_discrete.py.
• Validation performed locally on this branch, rebased onto current main:
  • python -m pytest -q pyomo/contrib/gdpopt/tests/ pyomo/contrib/mindtpy/tests/test_mindtpy_no_discrete.py
    • Result: 108 passed, 35 skipped, 5 deselected in 196.51s
  • 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: passed, 4 files would be left unchanged
  • typos --config ./.github/workflows/typos.toml on the four changed files
    • Result: passed, no findings

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Notes for reviewers (optional): This replacement PR carries over the implementation and tests from #3946 unchanged, then refreshes the branch against current main. The replacement PR description and branch-refresh workflow were prepared with AI assistance and reviewed before posting.

Three things I would like focused feedback on, since this PR is larger than the other GDPopt fixes I have open:

1. relaxed_nlp_solver defaults to None specifically so that nonconvex nodes keep their current global-bounding behavior. Please confirm that opt-in default is the semantics you want, rather than defaulting to the existing nlp_solver.
2. Extracting the GAMS reslim logic into _apply_rnGDP_subproblem_time_limit() means the local node screening path now also gets a GAMS time limit applied; previously that path passed local_minlp_solver_args through untouched. I believe that is the correct behavior, but it is a behavior change beyond the strict dispatch fix and should be a deliberate decision.
3. The MindtPy short-circuit change (len(discrete_variable_list) == 0 becoming "no unfixed discrete variables") alters when MindtPy short-circuits to a direct solve. It is needed for consistency with the LBB dispatch rule, but it touches MindtPy rather than GDPopt, so it may warrant a separate reviewer.

Related: this PR touches pyomo/contrib/gdpopt/tests/test_LBB.py near the same imports as #4010 (replacement for #3942, already approved by @emma58). The runtime changes are independent and do not overlap. I recommend merging #4010 first; I will then rebase this branch and resolve the small test-file conflict.

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