[Fleet] Integration policy doesn't get deleted if it is orphaned

[jen-huang]

Discovered while working on a case related to #129677.

If an integration policy is orphaned (its parent agent policy does not exist), this can cause various issues in the UI. The obvious solution is to bulk delete these policies using /api/fleet/package_policies/delete, but this does not work (even with force: true) due to that API attempting to update the non-existent agent policies. An example response:

[{"id":"dd177090-fdf5-4a2c-8cc1-5353a61f2b3d","success":false,"statusCode":404,"body":{"message":"Saved object [ingest-agent-policies/50cab1a0-aae0-11ec-aa7d-55640840aca0] not found"}},{"id":"917352cd-5933-414e-9c7b-a9fdddb9f262","success":false,"statusCode":404,"body":{"message":"Saved object [ingest-agent-policies/0f4f84d0-a1a8-11ec-87e9-8b3c1c1f01f5] not found"}}]

This API should be able to delete orphaned integration policies, perhaps after logging a warn that its parent agent policy doesn't exist.

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[juliaElastic]

Is this a duplicate of #134518?

[kpollich]

@juliaElastic No I don't think so. This issue seems to be about updating the DELETE API for package policies to not error in the case where a package policy's parent agent policy has been deleted. The other is to investigate the root cause of how we wind up in the scenario.

[kpollich]

This should be addressed by #182220. Closing in favor of the reusable policies work.