fix: fall back to regular backend discovery when manager API fails

The /rest/v2/manager/node/backends endpoint may not be supported on older
Doris versions. When getManagerBackendsByComputeGroup throws, fall back to
the regular /api/backends discovery instead of propagating the error.

Also broadened the catch in getManagerBackendsByComputeGroup from
ConnectedFailedException to Exception, so that parse errors (e.g. from
unexpected response formats on older versions) are also handled gracefully
by trying the next FE node.

Author: gnehil

flink-doris-connector/flink-doris-connector-base/src/main/java/org/apache/doris/flink/rest/RestService.java

@@ -354,8 +354,18 @@ public static List<BackendRowV2> getBackendsV2(
         }
 
         if (StringUtils.isNotBlank(computeGroupName)) {
-            return getManagerBackendsByComputeGroup(
-                    options, readOptions, feNodeList, computeGroupName, logger);
+            try {
+                return getManagerBackendsByComputeGroup(
+                        options, readOptions, feNodeList, computeGroupName, logger);
+            } catch (Exception e) {
+                logger.warn(
+                        "Failed to get backends via manager API for compute group '{}', "
+                                + "the endpoint {} may not be supported on older Doris versions. "
+                                + "Falling back to regular backend discovery: {}",
+                        computeGroupName,
+                        MANAGER_BACKENDS,
+                        e.getMessage());
+            }
         }
 
         for (String feNode : feNodeList) {
@@ -391,8 +401,8 @@ private static List<BackendRowV2> getManagerBackendsByComputeGroup(
                 String response = send(options, readOptions, httpGet, logger, false);
                 logger.info("Manager backend info:{}", response);
                 return parseManagerBackends(response, logger, computeGroupName);
-            } catch (ConnectedFailedException e) {
-                logger.info(
+            } catch (Exception e) {
+                logger.warn(
                         "Doris FE node {} is unavailable: {}, Request the next Doris FE node",
                         feNode,
                         e.getMessage());