Fix GLOA convex affine cut slope

[bernalde]

Fixes #3939.

Summary/Motivation:

GDPopt GLOA builds both of its affine cuts from a single cut_body expression computed from the MC++ concave slopes (ccSlope). The concave underestimator cut is correct, but the convex overestimator cut reuses those same concave slopes instead of the convex slopes (cvSlope). That produces an invalid upper cut, which can cut off feasible points and let GLOA terminate at a non-global solution.

MindtPy's GOA affine-cut path already uses the convex slope for its upper cut, so this brings GLOA in line with the existing correct implementation.

This is a replacement for #3940, which GitHub would not allow me to reopen after it was closed. The prior review discussion and history remain available there.

Integration evidence from GDPlib (recorded on #3940): on the GDPlib methanol GLOA benchmark, unpatched Pyomo 6.10.0 terminated after 1 iteration at objective -1743.4292381783366, while this branch ran 15 iterations and reached -1793.4292385308854, matching GDPlib's documented best-known objective -1793.4292381783 to solver tolerance. This was the focused methanol GLOA case; I did not run the full GDPlib benchmark matrix.

Changes proposed in this PR:

• Compute the concave and convex affine cut bodies separately in pyomo/contrib/gdpopt/gloa.py, so the convex overestimator cut uses cvSlope (MC++ subcv()) rather than reusing the concave ccSlope.
• Evaluate the trivially-True and trivially-False checks per cut, and add each cut independently, so a constant concave cut no longer suppresses a variable convex cut (and vice versa).
• Add test_gloa_affine_cut_uses_convex_slope_for_upper_cut to pyomo/contrib/gdpopt/tests/test_gdpopt.py, a solver-free regression covering the invalid-cut case from the issue.
• Add the matching test_goa_affine_cut_uses_convex_slope_for_upper_cut to pyomo/contrib/mindtpy/tests/unit_test.py, documenting that MindtPy GOA already uses the convex slope for its upper cut.
• Validation performed locally on this branch, rebased onto current main:
  • python -m pytest -q pyomo/contrib/gdpopt/tests/test_gdpopt.py pyomo/contrib/mindtpy/tests/unit_test.py
    • Result: 65 passed, 19 skipped, 3 deselected in 39.87s
  • python -m black --check pyomo/contrib/gdpopt/gloa.py pyomo/contrib/gdpopt/tests/test_gdpopt.py pyomo/contrib/mindtpy/tests/unit_test.py
    • Result: passed, 3 files would be left unchanged
  • typos --config ./.github/workflows/typos.toml on the three changed files
    • Result: passed, no findings

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Notes for reviewers (optional): This replacement PR carries over the implementation and tests from #3940 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. Reviewers may want to focus on the restructured trivial-cut handling: previously a single is_potentially_variable check governed both cuts together, and this PR splits that into independent per-cut checks, so the trivially-False DeveloperError conditions are now evaluated separately for the concave lower cut and the convex upper cut.

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