fix: ensure HTTP 503 responses are returned for failover processing instead of throwing ServerException

Author: kishansinghifs1

client-v2/src/main/java/com/clickhouse/client/api/internal/HttpAPIClientHelper.java

@@ -578,6 +578,8 @@ private ClassicHttpResponse doPostRequest(Map<String, Object> requestConfig, Htt
             } else if (httpResponse.getCode() == HttpStatus.SC_BAD_GATEWAY) {
                 httpResponse.close();
                 throw new ClientException("Server returned '502 Bad gateway'. Check network and proxy settings.");
+            } else if (httpResponse.getCode() == HttpStatus.SC_SERVICE_UNAVAILABLE) {
+                return httpResponse;
             } else if (httpResponse.getCode() >= HttpStatus.SC_BAD_REQUEST || httpResponse.containsHeader(ClickHouseHttpProto.HEADER_EXCEPTION_CODE)) {
                 try {
                     throw readError(req, httpResponse);