Use MultiProjectPendingException more consistently (#123955)

Use the same exception class in more places where single lookup fails
due to the existence of multiple projects. This allows the exception to
be handled similarly, e.g. ignored in
ClusterApplierService#callClusterStateListener

server/src/main/java/org/elasticsearch/cluster/metadata/Metadata.java

@@ -397,7 +397,7 @@ private Metadata updateSingleProject(Function<ProjectMetadata, ProjectMetadata>
                     reservedStateMetadata
                 );
         } else {
-            throw new UnsupportedOperationException("There are multiple projects " + projectMetadata.keySet());
+            throw new MultiProjectPendingException("There are multiple projects " + projectMetadata.keySet());
         }
     }
 
@@ -1310,7 +1310,7 @@ private ProjectMetadata.Builder getSingleProject() {
             if (projectMetadata.isEmpty()) {
                 createDefaultProject();
             } else if (projectMetadata.size() != 1) {
-                throw new UnsupportedOperationException("There are multiple projects " + projectMetadata.keySet());
+                throw new MultiProjectPendingException("There are multiple projects " + projectMetadata.keySet());
             }
             return projectMetadata.values().iterator().next();
         }

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

@@ -15,6 +15,7 @@
 import org.elasticsearch.cluster.DiffableUtils;
 import org.elasticsearch.cluster.DiffableUtils.KeySerializer;
 import org.elasticsearch.cluster.metadata.Metadata;
+import org.elasticsearch.cluster.metadata.Metadata.MultiProjectPendingException;
 import org.elasticsearch.cluster.metadata.ProjectId;
 import org.elasticsearch.cluster.metadata.ProjectMetadata;
 import org.elasticsearch.common.collect.ImmutableOpenMap;
@@ -134,7 +135,7 @@ public RoutingTable getRoutingTable() {
         return switch (routingTables.size()) {
             case 0 -> RoutingTable.EMPTY_ROUTING_TABLE;
             case 1 -> routingTables.values().iterator().next();
-            default -> throw new IllegalStateException("There are multiple project routing tables [" + routingTables.keySet() + "]");
+            default -> throw new MultiProjectPendingException("There are multiple project routing tables [" + routingTables.keySet() + "]");
         };
     }
 

server/src/main/java/org/elasticsearch/cluster/service/ClusterApplierService.java

@@ -594,7 +594,7 @@ private static void callClusterStateListener(
                     listener.clusterChanged(clusterChangedEvent);
                 }
             } catch (Metadata.MultiProjectPendingException e) {
-                // don't warn, this fills the logs
+                // don't warn, this fills the logs and also slow down applier thread in CI which could cause unrelated failures
                 logger.trace("ClusterStateListener not multi-project compatible", e);
             } catch (Exception ex) {
                 logger.warn("failed to notify ClusterStateListener", ex);