[Fleet] Create and update package policy API return 409 conflict when names are not unique

[criamico]

Closes #153026

Summary

The agent policy API returns a 409 in the case the policy already exists. The package policy API will now follow the same behavior.

Checklist

Delete any items that are not applicable to this PR.

• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios

[apmmachine]

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[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[kpollich]

Nice! Thanks for the refactor along the way 🚀

[juliaElastic]

nit: This logic could be simplified to use ES kuery to filter out the same id, and could use the total count without any hits.
Probably not a huge gain as there will be at least 1 matches, just some code simplification.

Something like:

  const existingPoliciesWithName = await packagePolicyService.list(soClient, {
    perPage: 0,
    kuery: `${PACKAGE_POLICY_SAVED_OBJECT_TYPE}.name:"${packagePolicy.name}" AND NOT (${PACKAGE_POLICY_SAVED_OBJECT_TYPE}.id:"${packagePolicy.id}")`,
  });
if (existingPoliciesWithName?.total > 0) {
  throw ...
}

[criamico]

Thanks Julia! Since the gain it's not very big I'm going to merge as it is, but we can do this change the next time we touch this area :)

[juliaElastic]

LGTM

[kibana-ci]

💚 Build Succeeded

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• Commit: f0c240f

Metrics [docs]

Unknown metric groups

ESLint disabled line counts

id	before	after	diff
securitySolution	433	436	+3

Total ESLint disabled count

id	before	after	diff
securitySolution	513	516	+3

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cc @criamico