fix: Remove BoundCylinderToZPhi final, add exception (#3926)

- I missed a `final` in one location (see #3923)
- Adding an exception to `GridAccessJsonConverter::toJson`. This was previously silently returning an empty json object if none of the downcasts worked. Since the downcast chain is not exhaustive, I think it's better to throw if there's an unhandled case.

Core/include/Acts/Utilities/GridAccessHelpers.hpp

@@ -212,7 +212,7 @@ class LocalSubspace : public IBoundToGridLocal {
   }
 };
 
-class BoundCylinderToZPhi final : public IBoundToGridLocal {
+class BoundCylinderToZPhi : public IBoundToGridLocal {
  public:
   double radius = 1.;
   double shift = 0.;

Plugins/Json/src/GridJsonConverter.cpp

@@ -439,6 +439,11 @@ nlohmann::json Acts::GridAccessJsonConverter::toJson(
     jBoundToGridLocal["shift"] = boundCylinderToZPhi->shift;
   }
 
+  if (jBoundToGridLocal.empty()) {
+    throw std::invalid_argument(
+        "GridAccessJsonConverter: boundToGridLocal type not supported.");
+  }
+
   return jBoundToGridLocal;
 }