fix: improve error handling in HttpClientSseClientTransport and add test

- Include response body in error messages for better debugging
- Change sendHttpPost return type from HttpResponse<Void> to HttpResponse<String>
- Use BodyHandlers.ofString() instead of discarding response body
- Add testErrorOnBogusMessage test to verify error handling for invalid JSON-RPC messages
- Test validates that malformed messages return 400 status with descriptive error message

Replace #361

Signed-off-by: Christian Tzolov <christian.tzolov@broadcom.com>

Co-authored-by: jkma <jkma@trip.com>

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

@@ -105,6 +105,16 @@ class HttpClientSseClientTransportTests {
 		container.stop();
 	}
 
+	@Test
+	void testErrorOnBogusMessage() {
+		// bogus message
+		JSONRPCRequest bogusMessage = new JSONRPCRequest(null, null, "test-id", Map.of("key", "value"));
+
+		StepVerifier.create(transport.sendMessage(bogusMessage))
+			.verifyErrorMessage(
+					"Sending message failed with a non-OK HTTP code: 400 - Invalid message: {\"id\":\"test-id\",\"params\":{\"key\":\"value\"}}");
+	}
+
 	@Test
 	void testMessageProcessing() {
 		// Create a test message