Prior to this commit, exceptions thrown by MVC functional handlers would
not be considered by `ExceptionHandlerExceptionResolver`. This means
that common exceptions would not be handled consistently between
annotated and functional handlers. This is true, for example, for all
`ProblemDetails`-related exception handling.
While MVC functional and annotation models are separate concepts,
WebFlux has a different error handling model that processes all
exceptions in a central place.
This commit ensures that `ExceptionHandlerExceptionResolver` considers
exceptions thrown by handlers of type `HandlerFunction<?>` and processes
them accordingly.
Closes gh-32689
52af43d6d2