Add case return_as_bytes to switches in detail/type_caster_base.h and eigen.h

Based on systematic review under #3838 (comment)

Author: rwgk

include/pybind11/detail/type_caster_base.h

@@ -651,6 +651,9 @@ class type_caster_generic {
                 keep_alive_impl(inst, parent);
                 break;
 
+            case return_value_policy::return_as_bytes:
+                pybind11_fail("return_value_policy::return_as_bytes does not apply.");
+
             default:
                 throw cast_error("unhandled return_value_policy: should not happen!");
         }

include/pybind11/eigen.h

@@ -349,6 +349,8 @@ struct type_caster<Type, enable_if_t<is_eigen_dense_plain<Type>::value>> {
                 return eigen_ref_array<props>(*src);
             case return_value_policy::reference_internal:
                 return eigen_ref_array<props>(*src, parent);
+            case return_value_policy::return_as_bytes:
+                pybind11_fail("return_value_policy::return_as_bytes does not apply.");
             default:
                 throw cast_error("unhandled return_value_policy: should not happen!");
         };