[Fleet] fixed bug in output secret diff logic

[juliaElastic]

Summary

Related to #104986

Found a bug in diffOutputSecretPaths where output secret was deleted if updating an output without change of service_token. Added unit tests to cover the logic.

Steps to verify:

• enable feature flags: xpack.fleet.enableExperimental: ['remoteESOutput', 'outputSecretsStorage']
• create a remote es output with a service_token
• check that the service_token is stored as secret in .fleet-secrets
• update host in remote es output
• verify that the secret is not deleted

Checklist

• Unit or functional tests were updated or added to match the most common scenarios

[elasticmachine]

Pinging @elastic/fleet (Team:Fleet)

[apmmachine]

🤖 GitHub comments

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• /oblt-deploy : Deploy a Kibana instance using the Observability test environments.
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[kpollich]

LGTM 🚀 - always good when the PR is 90% tests 🙂

[kibana-ci]

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: c57457c

Failed CI Steps

• Defend Workflows Cypress Tests on Serverless #4

Metrics [docs]

Async chunks

Total size of all lazy-loaded chunks that will be downloaded as the user navigates the app

id	before	after	diff
fleet	1.2MB	1.2MB	+8.0B

History

• 💛 Build #179628 was flaky 2ecd91f

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

cc @juliaElastic