Fix GDPopt LBB time-limit results

[bernalde]

Fixes #3941.

Summary/Motivation:

The GDPopt LBB time-limit path calls self._get_final_results_object(), which no longer exists. The method was renamed to _get_final_pyomo_results_object() during an earlier GDPopt rewrite, and this one call site was left stale. Any LBB solve that terminates by hitting its time limit raises AttributeError instead of returning a results object.

The same file already uses the current name at the other termination site (branch_and_bound.py:288), so this is an isolated leftover rather than an intentional difference.

This is a replacement for #3942, which GitHub would not allow me to reopen after it was closed. That PR was approved by @emma58 and was closed for a PR-template/CI-state reason rather than a code concern; the prior review discussion and history remain available there.

Changes proposed in this PR:

• Call _get_final_pyomo_results_object() on the GDPopt LBB time-limit termination path in pyomo/contrib/gdpopt/branch_and_bound.py, matching the other termination site in the same file.
• Add TestGDPopt_LBB_TimeLimit.test_time_limit_returns_pyomo_results_object, a solver-independent regression that forces the time-limit branch and asserts a Pyomo results object is returned with maxTimeLimit and unbounded problem bounds.
• Add TestMindtPyGOATimeLimit.test_goa_time_limit_sets_solver_results_condition, documenting that the corresponding MindtPy GOA time-limit path already sets its termination condition correctly.
• Validation performed locally on this branch, rebased onto current main:
  • python -m pytest -q pyomo/contrib/gdpopt/tests/test_LBB.py pyomo/contrib/mindtpy/tests/test_mindtpy_no_discrete.py
    • Result: 32 passed, 9 skipped, 2 deselected in 2.55s
  • python -m pytest -q pyomo/contrib/gdpopt/tests/
    • Result: 76 passed, 35 skipped, 5 deselected in 81.25s
  • python -m black --check --diff pyomo/contrib/gdpopt/branch_and_bound.py pyomo/contrib/gdpopt/tests/test_LBB.py pyomo/contrib/mindtpy/tests/test_mindtpy_no_discrete.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 #3942 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. The runtime change is a single call-site rename; reviewers may want to focus on whether the forced-time-limit regression test in test_LBB.py is the right way to exercise that path without requiring a MINLP subsolver, since it temporarily patches GDP_LBB_Solver.reached_time_limit and restores it in a finally block.

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