[Fleet] Accommodate fetching details about EPM custom integrations

[eokoneyo]

Kibana version:

https://github.com/elastic/kibana/tree/6c36503a6362435f17e7f2f3bdc0f69974d1e4c5 (this is the latest commit to main at the time of writing this issue, before the PR #174734 gets merged in).

Describe the bug:

Steps to reproduce:

1. Checkout main at the commit specified above
2. Run node scripts/build_kibana_platform_plugins --test-plugins --no-cache to ensure all files required to run the test are built
3. Next run node x-pack/plugins/observability_onboarding/scripts/test/e2e --server to start the ftr server then in a different terminal run node x-pack/plugins/observability_onboarding/scripts/test/e2e --open, this will start open the cypress dashboard
4. Choose to run E2E test, and select the install_elastic_agent.cy.ts spec.
5. You'll notice that all tests after the first one fails, this is because the endpoint epm/packages/{packageName} doesn't resolve custom integrations properly. The custom integration created in the first test is found in the saved object, but despite the fact that when inspecting saved objects this particular form of the integration has no assets as seen in value returned;

{
id: 'mylogs' type: 'epm-packages', 
namespaces: [], 
migrationVersion: undefined,
updated_at: '2024-01-11T19:51:36.577Z', 
created_at: '2024-01-11T19:51:36.121Z'
Version: 'WzExMywxXQ==',
attributes: {
  installed_kibana: [], 
  installed_kibana_space_id: 'default',
  installed_es: [Array],
  package_assets: [],
  es_index_patterns: [Object],
  name: 'mylogs',
  version: '1.0.0', 
  install_ version: 1.0.0'
  install_status: 'installed', 
  install_started_at: '2024-01-11719:51:36.121Z'
  install source: 'custom'
  install _format_schema_version: '1.1.0', 
  verification status: 'unknown', 
  latest_install_failed_attempts:
 }
}

An attempt is still to made to resolve it's assets, which in turn results in the following error on the server;

it's worth pointing out the same happens when the newer endpoint that expects the package version number is used

Expected behavior:

When custom integrations that are installed as packages with no assets are encountered, despite this we should skip trying to process it's non-existent assets.

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[weltenwort]

The fact that the custom integrations don't have assets is a bug introduced in #173965. I've created a fix in #174869. There should be no special handling necessary for these custom integrations, because they should have a valid structure.

[jen-huang]

@weltenwort Will #174869 fix this issue or is there still more to patch here?

[weltenwort]

the fix should be sufficient and has just been merged

[jen-huang]

Thanks @weltenwort, closing this one then.