Refactor cost access behind accessor object

[nathanneike]

Types of changes

• Refactoring / code cleanup
• Bug fix
• New feature
• Breaking change
• Documentation update

Motivation and context / Related issue

This PR cleans up cost handling in the network simplex solver by introducing a dedicated cost accessor object. The goal is to centralize cost retrieval for the different existing cost modes instead of keeping the logic duplicated across pricing, initialization, warmstart, and total-cost computation.

This does not aim to change solver behavior. It is a cleanup step to make the cost path easier to reason about and easier to extend in follow-up optimization work.

How has this been tested (if it applies)

• Rebuilt the compiled extension locally with clang:
  • CC=clang CXX=clang++ LDFLAGS="-L$PWD/.openmp/lib -Wl,-rpath,$PWD/.openmp/lib -lomp" .venv/bin/python setup.py build_ext --inplace
• Verified the modified C++ extension compiles successfully.
• Started the full pytest suite with:

PR checklist

• I have read the CONTRIBUTING document.
• The documentation is up-to-date with the changes I made (check build artifacts).
• All tests passed, and additional code has been covered with new tests.
• I have added the PR and Issue fix to the RELEASES.md file.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.87%. Comparing base (41a4d57) to head (475d179).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #804   +/-   ##
=======================================
  Coverage   96.87%   96.87%           
=======================================
  Files         113      113           
  Lines       23062    23062           
=======================================
  Hits        22342    22342           
  Misses        720      720           

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