Add setPreAuthenticationChecks to AbstractUserDetailsReactiveAuthenticationManager

Expected Behavior

AbstractUserDetailsReactiveAuthenticationManager should allow customizing the pre-authentication account status checks, the same way its servlet counterpart AbstractUserDetailsAuthenticationProvider does via setPreAuthenticationChecks(UserDetailsChecker).

Current Behavior

The reactive manager holds a private preAuthenticationChecks field and applies it in authenticate():

private UserDetailsChecker preAuthenticationChecks = this::defaultPreAuthenticationChecks;
...
return retrieveUser(username)
        .doOnNext(this.preAuthenticationChecks::check)

but only exposes setPostAuthenticationChecks. The field is private, the default implementation is a private method, and there is no protected getter — so a WebFlux application cannot customize how locked/disabled/expired accounts are checked (e.g. custom error messages or lockout policy) without overriding authenticate() entirely.

Context

The synchronous AbstractUserDetailsAuthenticationProvider exposes both setPreAuthenticationChecks and setPostAuthenticationChecks.

Start with AbstractUserDetailsReactiveAuthenticationManager.authenticate() and compare it with AbstractUserDetailsAuthenticationProvider.setPreAuthenticationChecks(UserDetailsChecker). Confirm that the reactive manager supports equivalent pre-authentication customization while retaining its default checks, and verify the relevant reactive authentication-manager tests.