Add WebClient integration test for multipart

This commit adds an integration test for `WebClient`, specifically
testing that a failure happening while pulishing the request body is
reported on the main reactive pipeline.

See gh-35678

spring-webflux/src/test/java/org/springframework/web/reactive/function/client/WebClientIntegrationTests.java

@@ -71,12 +71,14 @@ import org.springframework.http.HttpStatusCode;
 import org.springframework.http.MediaType;
 import org.springframework.http.ResponseCookie;
 import org.springframework.http.ResponseEntity;
+import org.springframework.http.client.MultipartBodyBuilder;
 import org.springframework.http.client.reactive.ClientHttpConnector;
 import org.springframework.http.client.reactive.HttpComponentsClientHttpConnector;
 import org.springframework.http.client.reactive.JdkClientHttpConnector;
 import org.springframework.http.client.reactive.JettyClientHttpConnector;
 import org.springframework.http.client.reactive.ReactorClientHttpConnector;
 import org.springframework.web.reactive.function.BodyExtractors;
+import org.springframework.web.reactive.function.BodyInserters;
 import org.springframework.web.reactive.function.client.WebClient.ResponseSpec;
 import org.springframework.web.testfixture.xml.Pojo;
 
@@ -1320,6 +1322,34 @@ class WebClientIntegrationTests {
 				.verify(Duration.ofSeconds(3));
 	}
 
+	@ParameterizedWebClientTest
+	void failWhileSendingMultipartRequest(ClientHttpConnector connector) throws IOException {
+		startServer(connector);
+		prepareResponse(response -> response
+				.setHeader("Content-Type", "text/plain").body("Hello"));
+
+		MultipartBodyBuilder data = new MultipartBodyBuilder();
+		data.part("first", "success");
+		data.asyncPart("second",
+				Mono.delay(Duration.ofMillis(500)).then(Mono.error(new IllegalStateException("multipart client error"))),
+				String.class);
+
+		Mono<ResponseEntity<String>> result = this.webClient.post()
+				.uri("/multipart").accept(MediaType.TEXT_PLAIN)
+				.body(BodyInserters.fromMultipartData(data.build()))
+				.retrieve()
+				.toEntity(String.class)
+				.log();
+
+		StepVerifier.create(result)
+				.expectErrorSatisfies(error -> assertThat(error)
+						.isInstanceOf(WebClientRequestException.class)
+						.hasMessageContaining("multipart client error"))
+				.verify(Duration.ofSeconds(5));
+
+		expectRequestCount(0);
+	}
+
 	private <T> Mono<T> doMalformedChunkedResponseTest(
 			ClientHttpConnector connector, Function<ResponseSpec, Mono<T>> handler) {