fix(gateway): reject oversized sparse array total_length with InvalidArgument

Guard against proto uint32 total_length values that exceed Array.MaxLength
before casting; the previous checked cast threw OverflowException (gRPC
Internal) instead of the intended InvalidArgument. Adds tests for the new
guard, for the null-value ArgumentNullException path, and removes the
checked keyword (redundant after the guard).

src/ZB.MOM.WW.MxGateway.Server/Sessions/SparseArrayExpander.cs

@@ -62,7 +62,13 @@ internal static class SparseArrayExpander
             throw Invalid($"Sparse array element_data_type '{elementType}' is not a supported scalar element type.");
         }
 
-        int length = checked((int)totalLength);
+        if (totalLength > (uint)Array.MaxLength)
+        {
+            throw Invalid(
+                $"Sparse array total_length {totalLength} exceeds the maximum supported array length {Array.MaxLength}.");
+        }
+
+        int length = (int)totalLength;
         HashSet<uint> seenIndices = new();
 
         foreach (MxSparseElement element in sparse.Elements)