Check the return code of every raw PETSc call in snessolver.cpp

Wrap every PETSc call, including inside the callback helper
functions, with CHECK(... == 0) for strict error checking, matching
the convention applied to la::petsc's own test suite.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_017uXpk1iyjECr5Eei7Sq1dH

Author: jhale