Wrap DecodingException in DefaultServerRequest
Prior to this commit, Spring WebFlux function would let
`DecodingException` thrown by codecs bubble up to the web handler level.
Since this exception is not handled by default there, the response would
be turned into a HTTP 500 status.
In the annotation model, `ArgumentResolver` implementations wrap this
exception with a `ServerWebInputException`, which itself extends
`ResponseStatusException`. The latter is supported by the error handling
infrastructure as a HTTP 400 response.
This commit ensures that `DecodingException` instances are properly
wrapped in `ServerWebInputException` at the `ServerRequest` level
directly, thus supporting all setup modes ("standalone" and through the