Fix no results returned when no discrete variables are present in MindtPy

[bernalde]

Fixes #3855 .

Fixes #3855

Summary/Motivation:

This PR fixes an issue where MindtPy can short-circuit on “no discrete decisions” (LP/NLP) and then fails to reliably return a proper SolverResults and/or load primal values onto the input model, even when the direct LP/NLP solve succeeds. This behavior breaks downstream meta-solvers (e.g., GDPopt subproblem solves) that depend on Var.value to capture an incumbent.

Reference: #3855

MindtPy contains a validation/short-circuit path intended to directly solve models that do not require decomposition (e.g., LP/NLP, or models where all discrete variables are fixed). In this path, MindtPy may:

return None from solve() (bare return)

Changes proposed in this PR:

• Ensure MindtPy always returns a SolverResults object (never None) even when the model is handled by the LP/NLP short-circuit.
• Initialize and populate self.results before early exits, and propagate termination/status/message from the direct LP/NLP solve into MindtPy’s results.
• Add a standalone test/test_mindtpy_no_discrete.py script

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[Toflamus]

Hi team,

I’ve investigated the current CI failures across Linux, macOS, and Windows. The failures in pyomo/contrib/solver/tests/solvers are not caused by the code changes in this PR.

The root cause is an expired GAMS license in the test environment. It appears the license might have expired a few days ago, which is why we are seeing identical failures across all platforms.

Interestingly, the tests for MindtPy are still passing (likely because they use different solver paths or have different fallback mechanisms), but the core solver tests are blocked.

Once the GAMS license is renewed in the CI environment, these tests should return to green.

[blnicho]

@Toflamus we are aware of the issue and discussed it during the developer call today. This is indeed an infrastructure issue and we're working on getting it fixed.