Address PR review: add ENSURE checks, fix duplicate test, fix comment

- Add ENSURE(result == Z3_L_TRUE) for each BNH optimization call and
  ENSURE(num_sat == 7) at the end so CI catches regressions.
- Remove test_bnh_optimize() from tst_api() to avoid duplicate
  execution under /a; keep standalone tst_bnh_opt() entry point.
- Fix Test 2 comment: it tests same-size backup, not backup-longer.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

src/test/lp/lp.cpp

@@ -1806,7 +1806,7 @@ void test_restore_x() {
         std::cout << "  test 1 (backup shorter): " << lp_status_to_string(status) << " - PASSED" << std::endl;
     }
 
-    // Test 2: backup longer than current (columns removed after backup, or pop)
+    // Test 2: same-size backup (restore_x copies all elements directly)
     {
         lar_solver solver;
         lpvar x = solver.add_var(0, false);