The DefaultIdentityLinkIterceptor.java creates duplicate identityLinks on ProcessInstance

**Describe the bug**
The DefaultIdentityLinkIterceptor.handleCompleteTask method creates a new PARTICIPANT IdentityLink on the ProcessInstance the tasks belongs to every time a task completes.

**Expected behavior**
There should be a check if such an IdentityLink already exists before creating it. This is done in some of the other methods in the class by calling the addUserIdentityLinkToParent which does such a check.

Noticed that we had a lot of duplicates in the IdentityLink-tables for processes with a lot of tasks.

**Code**
```
public void handleCompleteTask(TaskEntity task) {
if (Authentication.getAuthenticatedUserId() != null && task.getProcessInstanceId() != null) {
ExecutionEntity processInstanceEntity = CommandContextUtil.getExecutionEntityManager().findById(task.getProcessInstanceId());
IdentityLinkUtil.createProcessInstanceIdentityLink(processInstanceEntity,
Authentication.getAuthenticatedUserId(), null, IdentityLinkType.PARTICIPANT);
}
}
```

**Additional context**
This is present on the main branch.

DefaultIdentityLinkIterceptor.java の handleCompleteTask から始め、addUserIdentityLinkToParent を呼び出す周辺のメソッドとその動作を比較してください。既存の PARTICIPANT IdentityLink がどのように検出されるかを確認し、その後、同じプロセスインスタンスで複数のタスクを完了しても重複したリンクが作成されないことを確認してください。

索引モデルが issue の本文から書いたものです。