Add status handler to recognize unknown status codes outside of 4xx/5…

[duesenklipper]

…xx and throw an UnknownHttpStatusCodeException

org.springframework.web.reactive.function.client.UnknownHttpStatusCodeException claims in its Javadoc that it will be thrown to signal that "an unknown (or custom) HTTP status code is received". Up until Spring 5/Spring Boot 2 this was indeed true. We recently upgraded to Spring Boot 3/Spring 6, and our code that expected to receive this exception no longer works.

It turns out that the DEFAULT_STATUS_HANDLER in DefaultWebClient tests HttpStatusCode.isError as a predicate before calling DefaultClientResponse#createException. createException is the only place UnknownHttpStatusException is instantiated.

This is a behavior change from Spring 5, since now only 4xx/5xx custom status codes can lead to a UnknownHttpStatusException, in spite of what the exception's javadoc says.

This PR splits DEFAULT_STATUS_HANDLER into one for errors, and one for unknown codes. Both call the same function, with different predicates - one calls isError(), the other calls a new function isWellKnownStatusCode(). I kept the logic for both WebClientResponseException and UnknownHttpStatusCodeException in createException(), since there still can be a custom 4xx/5xx code. I also provide a test that validates the behavior for custom codes outside of that range, in addition to the existing test which only verified for a custom 5xx code.

[rstoyanchev]

It looks this may have happened inadvertently as part of changes to introduce the HttpStatusCode interface. I think we should simply revert to how the Predicate was before, i.e. checking for codes greater than or equal to 400. I don't see any benefit from the additional default handler.

@poutsma what do you think?

[rstoyanchev]

Tentatively scheduled for 6.1 but still pending team review.