[BUG] Fix variable fixing for initial solutions

[nguidotti]

When feeding initial solutions to the population, the diversity manager was fixing the variables on a default constructed problem. If the problem has free variables, it means that it was fixed to an infinity bound.

Checklist

• I am familiar with the Contributing Guidelines.
• Testing
  • New or existing tests cover these changes
  • Added tests
  • Created an issue to follow-up
  • NA
• Documentation
  • The documentation is up to date with these changes
  • Added new documentation
  • NA

[coderabbitai]

📝 Walkthrough

Walkthrough

MIP heuristic solution assignments are now seeded before completion LP processing, and fixed-variable assignments are checked for finite values before constraint updates.

Changes

MIP assignment handling

Layer / File(s)	Summary
Seed assignments before completion LP cpp/src/mip_heuristics/diversity/diversity_manager.cu	Copies the provided initial assignment into sol.assignment before LP setup and removes the later duplicate copy.
Validate fixed assignments cpp/src/mip_heuristics/problem/problem.cu	Adds a GPU-side finite-value assertion for assignments used to fix variables.

Estimated code review effort: 2 (Simple) | ~10 minutes

Suggested labels: improvement

Suggested reviewers: rg20, akifcorduk

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Title check	✅ Passed	The title is concise and accurately describes the main fix for initial solution variable fixing.
Description check	✅ Passed	The description matches the change by explaining the initial-solution fixing issue and the new guard for non-finite bounds.

✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests

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[github-actions]

CI Test Summary

✅ All 31 test job(s) passed.

[nguidotti]

/merge