tools/bilateralize: Add option for balance equality constraint in load_and_solve

[junukitashepard]

Add function to ensure balance equality in load_and_solve functions

What this PR does:

• Add ensure_balance_equality to remove the default import/export balance_equality sets and replace them with sets for trade technologies' "piped/shipped" levels. In short, this ensures that commodities are not getting stockpiled at the trade layer of the model. Users can choose whether to invoke.
• Add test for load_and_solve focused on this update; this test was written using Claude

PR checklist

• Continuous integration checks all ✅
• Add or expand tests; coverage checks both ✅

[codecov]

Codecov Report

❌ Patch coverage is 98.27586% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 74.2%. Comparing base (8e13506) to head (16ae1fc).

Files with missing lines	Patch %	Lines
...age_ix_models/tools/bilateralize/load_and_solve.py	93.7%	1 Missing ⚠️

Additional details and impacted files

@@          Coverage Diff          @@
##            main    #536   +/-   ##
=====================================
  Coverage   74.2%   74.2%           
=====================================
  Files        320     321    +1     
  Lines      25655   25713   +58     
=====================================
+ Hits       19047   19104   +57     
- Misses      6608    6609    +1     

Files with missing lines	Coverage Δ
...ls/tests/tools/bilateralize/test_load_and_solve.py	100.0% <100.0%> (ø)
...age_ix_models/tools/bilateralize/load_and_solve.py	61.2% <93.7%> (+3.3%)	⬆️

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