Gurobi solution pool tweak

[viens-code]

Fixes #3725

Summary/Motivation:

Adds the ability to do best effort solution pool collection

Changes proposed in this PR:

• Adds ability to change flag in solution pool to 1, best effort instead of defaulting to 2 in gurobi solution pool tools
• Running in best effort mode gives a warning that unexpected behavior may occur, per gurobi documentation: https://docs.gurobi.com/projects/optimizer/en/current/features/solutionpool.html

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[viens-code]

Is this a my code issue or an issue with win/3.9 and 3.13?
It didn't get to the pyomo tests and my only changes were in contrib.alternative_solutions

[mrmundt]

@viens-code - neither were related to your code. The 3.9 one was a spurious failure; the 3.13 one is now just happening everywhere (yay broken infrastructure). I'm looking into it.

[jsiirola]

One requested change for a test, but more fundamentally, why is this needed? Can't users get exactly the same behavior with

gurobi_generate_solutions(..., solver_options={'PoolSearchMode': 2})

[jsiirola]

This is a weak test: not only will any assertion that fails pass, but not raising any error at all will also pass. The recommended / standard way to test an error is to check both the error type and the message:

with self.assertRaisesRegex(AssertionError, "pool_search_mode must be 1 or 2"):
    gurobi_generate_solutions(m, pool_search_mode=0)

[viens-code]

Changed over to the suggested format

[viens-code]

First, will make the tweaks to the testing logic to update to your suggestion, thank you for that.

To the larger point, there are a couple of things going on with the alternative_solutions package.
One of which is trying to make the alternative solutions methods easier to use for people who couldn't/wouldn't write the custom functionality for themselves.

Yes an advanced user could change from enforced enumerate (mode 2) to best effort (mode 1) with the manual flag if they knew about it.
The functionality add here was about adding best-effort solution discovery as a clear option in contrib.alternative_solutions.

It also parallels how things like num_solutions are handled as parameters outside solver_options.
We could do solver_options={'PoolSolutions': num_solutions} instead there too.

[blnicho]

@viens-code as you're working on additional changes, please make sure to merge main into your branch. This will resolve the win/3.13 failure which is coming from a testing infrastructure issue.

[viens-code]

Changes to that test structure done

[viens-code]

@mrmundt @blnicho
So we seem to have different failures on win/3.10, are these also just Pyomo CI issues like before?

[blnicho]

@viens-code seems like it was a temporary CI issue. Re-running the win/3.10 job seems to have resolved it.

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 89.31%. Comparing base (e173cb5) to head (b65b5e0).

Files with missing lines	Patch %	Lines
pyomo/contrib/alternative_solutions/solnpool.py	85.71%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3726      +/-   ##
==========================================
- Coverage   89.31%   89.31%   -0.01%     
==========================================
  Files         896      896              
  Lines      103682   103688       +6     
==========================================
+ Hits        92605    92610       +5     
- Misses      11077    11078       +1     

Flag	Coverage Δ
builders	29.10% <0.00%> (+<0.01%)	⬆️
default	85.93% <85.71%> (?)
expensive	35.86% <0.00%> (?)
linux	86.96% <85.71%> (-2.09%)	⬇️
linux_other	86.96% <85.71%> (-0.01%)	⬇️
osx	83.10% <85.71%> (+<0.01%)	⬆️
win	85.21% <85.71%> (-0.01%)	⬇️
win_other	85.21% <85.71%> (-0.01%)	⬇️

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