[Fleet] Unable to reassign agents to another policy when current policy is deleted

[jen-huang]

Originally posted by @patrykkopycinski in #89372 (comment)

I've found one more issue I think, I've had a case where my agents were assigned to the policy that was removed and when I tried to reassign agents to the new policy I was getting an error that saved object with old policy id doesn't exist
https://github.com/elastic/kibana/pull/89372/files#diff-11bcc5bb1eaf5ddae753f9ee690233bef988d13ddbf6baf71c4c8edd0d6a4933L186-L189

I've had to wrap this line into try/catch block to make sure that saved object error is not being thrown

let agentPolicySO;

try {
  agentPolicySO = await soClient.get<AgentPolicySOAttributes>(SAVED_OBJECT_TYPE, id);
  // eslint-disable-next-line no-empty
} catch (e) {}

if (!agentPolicySO) {
  return null;
}

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[afgomez]

I cannot reproduce the issue as described, since the UI doesn't allow to remove a policy if it has an agent assigned. However I manged to get a similar error with the following steps

1. Create a policy
2. Enroll an agent, and assign the created policy
3. Unenroll the agent
4. Delete the policy: get the error on fleet/agent_policies/delete

I'm going to tackle that scenario