feat(http): also retry on transient status code 520 (#337)

Add Cloudflare status code 520 ("Web Server Returned an Unknown Error")
to the HTTP client's retryable status codes, alongside the existing
503/521/522/524/530.

Brings this SDK in line with the go-sdk and python-sdk. Relates to
descope/etc#14039

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: dorsha

src/main/java/com/descope/proxy/impl/AbstractProxyImpl.java

@@ -36,12 +36,13 @@ abstract class AbstractProxyImpl {
 
   // HTTP status codes that should trigger automatic retries:
   // 503: Service Unavailable
+  // 520: Web Server Returned an Unknown Error (Cloudflare)
   // 521: Web Server Is Down (Cloudflare)
   // 522: Connection Timed Out (Cloudflare)
   // 524: A Timeout Occurred (Cloudflare)
   // 530: Cloudflare error
   static final Set<Integer> retryableStatusCodes = Collections.unmodifiableSet(
-      new HashSet<>(Arrays.asList(503, 521, 522, 524, 530)));
+      new HashSet<>(Arrays.asList(503, 520, 521, 522, 524, 530)));
 
   // Retry delays in milliseconds: first retry after 100ms, subsequent retries after 5000ms.
   // Package-private to allow overriding in tests.

src/test/java/com/descope/proxy/impl/AbstractProxyImplTest.java

@@ -55,17 +55,18 @@ void testRetryDelayConfig() {
   @Test
   void testRetryStatusCodesConfig() {
     assertTrue(AbstractProxyImpl.retryableStatusCodes.contains(503));
+    assertTrue(AbstractProxyImpl.retryableStatusCodes.contains(520));
     assertTrue(AbstractProxyImpl.retryableStatusCodes.contains(521));
     assertTrue(AbstractProxyImpl.retryableStatusCodes.contains(522));
     assertTrue(AbstractProxyImpl.retryableStatusCodes.contains(524));
     assertTrue(AbstractProxyImpl.retryableStatusCodes.contains(530));
-    assertEquals(5, AbstractProxyImpl.retryableStatusCodes.size());
+    assertEquals(6, AbstractProxyImpl.retryableStatusCodes.size());
   }
 
   @Test
   @SuppressWarnings({"unchecked", "rawtypes"})
   void testRetryOnRetryableStatusCodes() throws IOException {
-    List<Integer> retryableCodes = Arrays.asList(503, 521, 522, 524, 530);
+    List<Integer> retryableCodes = Arrays.asList(503, 520, 521, 522, 524, 530);
     for (int statusCode : retryableCodes) {
       AtomicInteger callCount = new AtomicInteger(0);
       CloseableHttpClient mockClient = mock(CloseableHttpClient.class);