Always forward entity merge requests from perfStandby (#24325)

Update requests to /sys/identity/entity/merge perform merges on perfStandby nodes in memory and skip the persist call.

This commit changes the behavior for the merge endpoint, forcing it to be forwarded from the standby to the active node. This change is specifically scoped to manual merges, as automatic merges are not isolated to a specific endpoint and require careful consideration for all callers.

changelog/24325.txt

@@ -0,0 +1,4 @@
+```release-note:change
+identity (enterprise): POST requests to the `/identity/entity/merge` endpoint
+are now always forwarded from standbys to the active node.
+```

vault/identity_store_entities.go

@@ -232,6 +232,7 @@ func entityPaths(i *IdentityStore) []*framework.Path {
 			Operations: map[logical.Operation]framework.OperationHandler{
 				logical.UpdateOperation: &framework.PathOperation{
 					Callback:                  i.pathEntityMergeID(),
+					ForwardPerformanceStandby: true,
 				},
 			},