SpringBoot Webflux actuator endpoints respond with 500 when a parameter is missing

Take this endpoint:

@Endpoint(id = "test")
@Component
public class TestEndpoint {
    @ReadOperation
    public Mono<String> sessionsForUsername(String username) {
        return Mono.just("test");
    }
}

When I execute curl http://localhost:8080/actuator/test, I get

HTTP/1.1 500 Internal Server Error
Content-Type: application/json
Content-Length: 139

{
    "timestamp": "2024-01-18T10:44:27.123+00:00",
    "path": "/actuator/test",
    "status": 500,
    "error": "Internal Server Error",
    "requestId": "3076ac2f-2"
}

When I remove the Mono, or I use WebMVC, i get a 400 Bad Request, like i would have expected it.

HTTP/1.1 400 
Content-Type: application/json
Transfer-Encoding: chunked
Date: Thu, 18 Jan 2024 10:45:36 GMT
Connection: close

{
    "timestamp": "2024-01-18T10:45:36.289+00:00",
    "status": 400,
    "error": "Bad Request",
    "path": "/actuator/test"
}

Reproducer: repro.zip

Comment From: scottfrederick

Closing in favor of #39397