test(spring-boot): Return a Mono from the Boot 2 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/src/integrationTestWebflux/kotlin/io/sentry/spring/boot/webfluxintegrationtest/GreetingController.kt

@@ -3,15 +3,18 @@ package io.sentry.spring.boot.webfluxintegrationtest
 import org.springframework.graphql.data.method.annotation.Argument
 import org.springframework.graphql.data.method.annotation.QueryMapping
 import org.springframework.stereotype.Controller
+import reactor.core.publisher.Mono
 
 /** Throws when given "crash", so tests can check error reporting. */
 @Controller
 open class GreetingController {
+  // Returns a Mono, as a reactive app would, so the test covers Sentry finishing the field's span
+  // asynchronously.
   @QueryMapping
-  open fun greeting(@Argument name: String): String {
+  open 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!")
   }
 }