Track pre-existing LP unbounded status misclassification in B&B fallback mapping

[coderabbitai]

A pre-existing status-mapping issue was identified in the branch-and-bound fallback LP status conversion path.

Context:

• The conversion helper in cpp/src/branch_and_bound/branch_and_bound.cpp maps lp_status_t values into dual-simplex status values for fallback solve paths.
• lp_status_t::UNBOUNDED is not handled explicitly and can fall through to the generic numerical-status path.
• This can cause an unbounded LP status to be reported or handled as a numerical issue in affected fallback paths.

Backlinks:

• PR: Refactor: mathematical_optimization namespace #1446
• Review discussion: Refactor: mathematical_optimization namespace #1446 (comment)

Requested by: @mlubin