Fix invalid request handling for WebFlux actuator endpoints

Fixes gh-39236

spring-boot-project/spring-boot-actuator/src/test/java/org/springframework/boot/actuate/endpoint/web/annotation/AbstractWebEndpointIntegrationTests.java

@@ -226,6 +226,31 @@ public abstract class AbstractWebEndpointIntegrationTests<T extends Configurable
 					.isEqualTo("1 2"));
 	}
 
+	@Test
+	void readOperationWithQueryParametersMissing() {
+		load(QueryEndpointConfiguration.class,
+				(client) -> client.get().uri("/query").exchange().expectStatus().isBadRequest());
+	}
+
+	@Test
+	void reactiveReadOperationWithSingleQueryParameters() {
+		load(ReactiveQueryEndpointConfiguration.class,
+				(client) -> client.get()
+					.uri("/query?param=test")
+					.exchange()
+					.expectStatus()
+					.isOk()
+					.expectBody()
+					.jsonPath("query")
+					.isEqualTo("test"));
+	}
+
+	@Test
+	void reactiveReadOperationWithQueryParametersMissing() {
+		load(ReactiveQueryEndpointConfiguration.class,
+				(client) -> client.get().uri("/query").exchange().expectStatus().isBadRequest());
+	}
+
 	@Test
 	void readOperationWithSingleQueryParametersAndMultipleValues() {
 		load(QueryEndpointConfiguration.class,
@@ -732,6 +757,17 @@ public abstract class AbstractWebEndpointIntegrationTests<T extends Configurable
 
 	}
 
+	@Configuration(proxyBeanMethods = false)
+	@Import(BaseConfiguration.class)
+	static class ReactiveQueryEndpointConfiguration {
+
+		@Bean
+		ReactiveQueryEndpoint reactiveQueryEndpoint() {
+			return new ReactiveQueryEndpoint();
+		}
+
+	}
+
 	@Configuration(proxyBeanMethods = false)
 	@Import(BaseConfiguration.class)
 	static class VoidWriteResponseEndpointConfiguration {
@@ -974,6 +1010,16 @@ public abstract class AbstractWebEndpointIntegrationTests<T extends Configurable
 
 	}
 
+	@Endpoint(id = "query")
+	static class ReactiveQueryEndpoint {
+
+		@ReadOperation
+		Mono<Map<String, String>> query(String param) {
+			return Mono.just(Collections.singletonMap("query", param));
+		}
+
+	}
+
 	@Endpoint(id = "voidwrite")
 	static class VoidWriteResponseEndpoint {