Fix HTTP status error template rendering in WebFlux

Prior to this commit, a change in `HttpStatus.toString` since SPR-16898
prevented the default WebFlux `ErrorWebExceptionHandler` to render
template views for exact HTTP status (e.g. "404.html").
This issue does not affect the resolution of series, like "4xx.html".

This commit fixes `DefaultErrorWebExceptionHandler` to use
`HttpStatus.value()` when attempting to resolve error views.

Closes gh-15083

spring-boot-project/spring-boot-autoconfigure/src/main/java/org/springframework/boot/autoconfigure/web/reactive/error/DefaultErrorWebExceptionHandler.java

@@ -122,7 +122,7 @@ public class DefaultErrorWebExceptionHandler extends AbstractErrorWebExceptionHa
 		ServerResponse.BodyBuilder responseBody = ServerResponse.status(errorStatus)
 				.contentType(MediaType.TEXT_HTML);
 		return Flux
-				.just("error/" + errorStatus.toString(),
+				.just("error/" + errorStatus.value(),
 						"error/" + SERIES_VIEWS.get(errorStatus.series()), "error/error")
 				.flatMap((viewName) -> renderErrorView(viewName, responseBody, error))
 				.switchIfEmpty(this.errorProperties.getWhitelabel().isEnabled()

spring-boot-project/spring-boot-autoconfigure/src/test/java/org/springframework/boot/autoconfigure/web/reactive/error/DefaultErrorWebExceptionHandlerIntegrationTests.java

@@ -257,6 +257,43 @@ public class DefaultErrorWebExceptionHandlerIntegrationTests {
 				});
 	}
 
+	@Test
+	public void exactStatusTemplateErrorPage() {
+		this.contextRunner
+				.withPropertyValues("server.error.whitelabel.enabled=false",
+						"spring.mustache.prefix=" + getErrorTemplatesLocation())
+				.run((context) -> {
+					WebTestClient client = WebTestClient.bindToApplicationContext(context)
+							.build();
+					String body = client.get().uri("/notfound")
+							.accept(MediaType.TEXT_HTML).exchange().expectStatus()
+							.isNotFound().expectBody(String.class).returnResult()
+							.getResponseBody();
+					assertThat(body).contains("404 page");
+				});
+	}
+
+	@Test
+	public void seriesStatusTemplateErrorPage() {
+		this.contextRunner
+				.withPropertyValues("server.error.whitelabel.enabled=false",
+						"spring.mustache.prefix=" + getErrorTemplatesLocation())
+				.run((context) -> {
+					WebTestClient client = WebTestClient.bindToApplicationContext(context)
+							.build();
+					String body = client.get().uri("/badRequest")
+							.accept(MediaType.TEXT_HTML).exchange().expectStatus()
+							.isBadRequest().expectBody(String.class).returnResult()
+							.getResponseBody();
+					assertThat(body).contains("4xx page");
+				});
+	}
+
+	private String getErrorTemplatesLocation() {
+		String packageName = getClass().getPackage().getName();
+		return "classpath:/" + packageName.replace('.', '/') + "/templates/";
+	}
+
 	@Test
 	public void invalidAcceptMediaType() {
 		this.contextRunner.run((context) -> {

spring-boot-project/spring-boot-autoconfigure/src/test/resources/org/springframework/boot/autoconfigure/web/reactive/error/templates/error/404.mustache

@@ -0,0 +1 @@
+404 page

spring-boot-project/spring-boot-autoconfigure/src/test/resources/org/springframework/boot/autoconfigure/web/reactive/error/templates/error/4xx.mustache

@@ -0,0 +1 @@
+4xx page