Fix non-OK HTTP POST responses handling (#374)

Currently, JDK HttpClient SSE implementation incorrectly handles a
non-2xx HTTP response code for POST. Instead of immediately failing the
caller, it ignores the issue and the user awaits until the timeout kicks
in.

Set HttpClient to use HTTP/1.1 version explicitly for tests

A related problem happened with the WebClient Streamable HTTP
implementation, where the error was swallowed for non-400 responses.

Signed-off-by: Dariusz Jędrzejczyk <dariusz.jedrzejczyk@broadcom.com>

mcp/src/test/java/io/modelcontextprotocol/client/transport/HttpClientSseClientTransportTests.java

@@ -59,7 +59,9 @@ class HttpClientSseClientTransportTests {
 		private Sinks.Many<ServerSentEvent<String>> events = Sinks.many().unicast().onBackpressureBuffer();
 
 		public TestHttpClientSseClientTransport(final String baseUri) {
-			super(HttpClient.newHttpClient(), HttpRequest.newBuilder(), baseUri, "/sse", new ObjectMapper());
+			super(HttpClient.newBuilder().version(HttpClient.Version.HTTP_1_1).build(),
+					HttpRequest.newBuilder().header("Content-Type", "application/json"), baseUri, "/sse",
+					new ObjectMapper());
 		}
 
 		public int getInboundMessageCount() {