Guard BandGeneral solver against invalid LAPACK inputs

[masaqif]

Summary

• validate BandGeneral bandwidth and LAPACK leading-dimension storage before calling DGBSV/DGBTRS
• reject incomplete solver storage and non-finite matrix or right-hand-side values with an OpenSees error
• add OpenSeesPy regressions covering non-finite matrix and right-hand-side input

This addresses the invalid-input failure mode reported in #1611 by preventing malformed data from reaching the Fortran LAPACK routine.

Verification

• cmake --build build/regression --target OpenSeesPy -j4
• python tests/run_pytest.py --module build/regression/OpenSeesPy.dylib --tests-dir tests/test_bandgen_solver.py
• result: 2 passed

Contributor: Musawer Ahmad Saqif saqif@umich.com