When `EnvironmentEndpoint` is building a response to return to the
web infrastructure, it creates a data structure containing all
property values from all property sources. Prior to this commit, it
was possible for the response data structure to contain property
values that were not serializable to JSON by Jackson, which would
cause an exception to be thrown by the web infrastructure. This
commit ensures the data structure is serializable to JSON by
ensuring property values are primitives or Strings, and returning
a placeholder value if a property value is of any other type.

Fixes gh-23805