test(spring-boot): Return a Mono from the Boot 4 WebFlux GraphQL resolver

The sample's resolver returned a Mono, so its test covered Sentry
finishing the field's span when the Mono completes. The port returned a
String, which only covers the synchronous path.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

Author: runningcode

sentry-spring-boot-4/src/integrationTestWebflux/kotlin/io/sentry/spring/boot4/webfluxintegrationtest/WebfluxIntegrationTestApplication.kt

@@ -94,11 +94,13 @@ class TodoController(private val webClient: WebClient) {
 /** Throws when given "crash", so tests can check error reporting. */
 @Controller
 class GreetingController {
+  // Returns a Mono, as a reactive app would, so the test covers Sentry finishing the field's span
+  // asynchronously.
   @QueryMapping
-  fun greeting(@Argument name: String): String {
+  fun greeting(@Argument name: String): Mono<String> {
     if (name == "crash") {
       throw RuntimeException("causing an error for $name")
     }
-    return "Hello $name!"
+    return Mono.just("Hello $name!")
   }
 }