Fixes based on resharding disruption tests (elastic#130870)

Author: lkts

server/src/main/java/org/elasticsearch/cluster/routing/RecoverySource.java

@@ -355,7 +355,7 @@ protected void writeAdditionalFields(StreamOutput out) throws IOException {
 
         @Override
         public void addAdditionalFields(XContentBuilder builder, Params params) throws IOException {
-            sourceShardId.toXContent(builder, params);
+            builder.field("sourceShardId", sourceShardId);
         }
     }
 }

server/src/main/java/org/elasticsearch/indices/cluster/IndicesClusterStateService.java

@@ -1004,8 +1004,8 @@ private static DiscoveryNode findSourceNodeForReshardSplitRecovery(
         ShardRouting sourceShardRouting = routingTable.shardRoutingTable(sourceShardId).primaryShard();
 
         if (sourceShardRouting.active() == false) {
-            assert false : sourceShardRouting.shortSummary();
-            logger.trace("can't find reshard split source node because source shard {} is not active.", sourceShardRouting);
+            // Source shard is unassigned (likely due to failure), we will retry.
+            logger.trace("can't find reshard split source node because source shard {} is not active.", sourceShardRouting.shortSummary());
             return null;
         }