[Fleet] Fix is_managed behavior in preconfiguration service

[nchaulet]

Summary

Related to #113921

When an agent policy that was not managed become managed the preconfiguration service do not take this in account.
Also with the current implementation of the preconfiguration service we add the is_managed:true flag to a policy not during the creation of the agent policy Saved object but at the end of the preconfiguration, so if an error happen the preconfigured policy will never be updated,= never be a managed policy and some package policy could be missing (I think this happened a few time in cloud).

How to tests/Reproduce the bug

1. add a policy to your kibana.dev.yml

xpack.fleet.agentPolicies:
  - name: Test agent policy
    id: test-policy
    monitoring_enabled: []
    package_policies: []

1. navigate to fleet and check the policy is created
2. Update the policy in your kibana.dev.yml

xpack.fleet.agentPolicies:
  - name: Test agent policy
    is_managed: true
    id: test-policy
    monitoring_enabled: []
    # data_output_id: output-123
    # monitoring_output_id: output-123
    package_policies:
      - package:
          name: fleet_server
        name: Fleet Server
        inputs:
          - type: fleet-server
            keep_enabled: true

1. with my fix when you navigate to kibana you should the policy as managed (with the fleet server package policy. , not working for now)

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[kibanamachine]

💔 Build Failed

• continuous-integration/kibana-ci/pull-request
• Commit: b46da85
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• Pipeline Steps (look for red circles / failed steps)
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Failed CI Steps

• Jest Unit Tests

---

Test Failures

Kibana Pipeline / jest / Jest Tests.x-pack/plugins/observability/public/context.HasDataContextProvider when plugins have registered all apps return false hasAnyData returns false and all apps return false

Link to Jenkins

Standard Out

Failed Tests Reporter:
  - Test has not failed recently on tracked branches

Stack Trace

Error: Timed out in waitForNextUpdate after 1000ms.
    at waitForNextUpdate (/var/lib/jenkins/workspace/elastic+kibana+pipeline-pull-request/kibana/node_modules/@testing-library/react-hooks/lib/core/asyncUtils.js:102:13)
    at /var/lib/jenkins/workspace/elastic+kibana+pipeline-pull-request/kibana/x-pack/plugins/observability/public/context/has_data_context.test.tsx:139:11

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Metrics [docs]

✅ unchanged

To update your PR or re-run it, just comment with:
@elasticmachine merge upstream

cc @nchaulet