[Fleet] policy_revision_idx is not updated on policy change ack for policy without Elasticsearch output

[nchaulet]

Description

Looks like policy_revision_idx is not updated in .fleet-agents for policy without Elasticsearch output, normally when an agent acknowledge a policy change fleet server update the policy_revision_idx looks like for policy with only logstash output (for data and monitoring, and agent never used ES before) the field is not updated

Impact

Edit: On Kibana > Fleet > Agents, the impacted agents, the ones with all outputs set to Logstash, show as Updating instead of Health and no revision is shown in the UI as well.

I think the problem was introduced in that commit ca9041f and impact 8.5+ agents

Details

In the code that handle the action acknowledgement

fleet-server/internal/pkg/api/handleAck.go

Line 587 in 7b0d25a

func makeUpdatePolicyBody(policyID string, newRev, coordIdx int64) []byte {

the function makeUpdatePolicyBody that update the policy_revision_idx is now called in the updateApiKey if the policy output is elasticsearch in this case look like the only output is logstash this will explain why it's not udpated

I think the problem was introduced in that commit ca9041f

How to reproduce

1. create an agent policy that use a logstash output for both data and monitoring
2. enroll an agent
3. create a new revision of the policy previously created (you can see that policy_revision_idx is not populated and the UI do not display the revision)