fix captcha shortening url path resolution

service/src/main/java/org/whispersystems/textsecuregcm/captcha/ShortCodeExpander.java

@@ -28,7 +28,7 @@ public ShortCodeExpander(final HttpClient client, final String shortenerHost) {
   }
 
   public Optional<String> retrieve(final String shortCode) throws IOException {
-    final URI uri = shortenerHost.resolve("/" + shortCode);
+    final URI uri = shortenerHost.resolve(shortCode);
     final HttpRequest request = HttpRequest.newBuilder().uri(uri).GET().build();
 
     try {

service/src/test/java/org/whispersystems/textsecuregcm/captcha/ShortCodeExpanderTest.java

@@ -0,0 +1,72 @@
+/*
+ * Copyright 2023 Signal Messenger, LLC
+ * SPDX-License-Identifier: AGPL-3.0-only
+ */
+
+package org.whispersystems.textsecuregcm.captcha;
+
+import com.google.api.Http;
+import org.junit.jupiter.api.Test;
+import org.mockito.ArgumentMatchers;
+import javax.net.ssl.SSLSession;
+import java.io.IOException;
+import java.net.URI;
+import java.net.http.HttpClient;
+import java.net.http.HttpHeaders;
+import java.net.http.HttpRequest;
+import java.net.http.HttpResponse;
+import java.util.Optional;
+
+import static org.assertj.core.api.Assertions.assertThat;
+import static org.mockito.ArgumentMatchers.any;
+import static org.mockito.ArgumentMatchers.argThat;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.when;
+
+public class ShortCodeExpanderTest {
+
+  @Test
+  public void testUriResolution() throws IOException, InterruptedException {
+    final HttpClient httpClient = mock(HttpClient.class);
+    final ShortCodeExpander expander = new ShortCodeExpander(httpClient, "https://www.example.org/shortener/");
+    when(httpClient
+        .send(argThat(req -> req.uri().toString().equals("https://www.example.org/shortener/shorturl")), any()))
+        .thenReturn(new FakeResponse(200, "longurl"));
+    assertThat(expander.retrieve("shorturl").get()).isEqualTo("longurl");
+  }
+
+  private record FakeResponse(int statusCode, String body) implements HttpResponse<Object> {
+
+    @Override
+    public HttpRequest request() {
+      return null;
+    }
+
+    @Override
+    public Optional<HttpResponse<Object>> previousResponse() {
+      return Optional.empty();
+    }
+
+    @Override
+    public HttpHeaders headers() {
+      return null;
+    }
+
+    @Override
+    public Optional<SSLSession> sslSession() {
+      return Optional.empty();
+    }
+
+    @Override
+    public URI uri() {
+      return null;
+    }
+
+    @Override
+    public HttpClient.Version version() {
+      return null;
+    }
+
+  }
+
+}