Check cudss_loaded() and error instead of falling back to Krylov

Updated defaultalg() to check if CUDSS is loaded and error immediately
if not available, rather than removing the check entirely or falling
back to Krylov (which also doesn't work on GPU).

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: ChrisRackauckas

ext/LinearSolveCUDAExt.jl

@@ -22,12 +22,20 @@ end
 
 function LinearSolve.defaultalg(A::CUDA.CUSPARSE.CuSparseMatrixCSR{Tv, Ti}, b,
         assump::OperatorAssumptions{Bool}) where {Tv, Ti}
-    LinearSolve.DefaultLinearSolver(LinearSolve.DefaultAlgorithmChoice.LUFactorization)
+    if LinearSolve.cudss_loaded(A)
+        LinearSolve.DefaultLinearSolver(LinearSolve.DefaultAlgorithmChoice.LUFactorization)
+    else
+        error("CUDSS.jl is required for LU Factorizations on CuSparseMatrixCSR. Please load this library.")
+    end
 end
 
 function LinearSolve.defaultalg(A::CUDA.CUSPARSE.CuSparseMatrixCSC{Tv, Ti}, b,
         assump::OperatorAssumptions{Bool}) where {Tv, Ti}
-    LinearSolve.DefaultLinearSolver(LinearSolve.DefaultAlgorithmChoice.LUFactorization)
+    if LinearSolve.cudss_loaded(A)
+        LinearSolve.DefaultLinearSolver(LinearSolve.DefaultAlgorithmChoice.LUFactorization)
+    else
+        error("CUDSS.jl is required for LU Factorizations on CuSparseMatrixCSC. Please load this library.")
+    end
 end
 
 function LinearSolve.error_no_cudss_lu(A::CUDA.CUSPARSE.CuSparseMatrixCSR)