[AO] Unskip the custom threshold rule executor unit test

[maryam-saeidi]

Fixes #162299
Fixes #162540

Summary

This PR unskips the custom threshold rule executor unit test and removes the warning implementation from the BE.

🧪 How to test

• Create a custom threshold rule, it should work as before. (Warning implementation logic was already removed from FE; this PR only removes the BE implementation.)

[apmmachine]

🤖 GitHub comments

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

💛 Build succeeded, but was flaky

• Buildkite Build
• Commit: 23c9cf5

Failed CI Steps

• FTR Configs #14
• FTR Configs #56

Test Failures

• [job] [logs] FTR Configs #56 / Profiling API tests has_setup.spec.ts cloud Loading profiling data Profiling status check APM integration is not installed Admin user has been set up
• [job] [logs] FTR Configs #14 / serverless common UI Index Management Index Templates "before all" hook for "renders the index templates tab"

Metrics [docs]

✅ unchanged

History

• 💔 Build #161724 failed ef40cfc
• 💔 Build #161629 failed 944ab3c
• 💔 Build #161580 failed ee95f52

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

cc @maryam-saeidi

[mgiota]

Unrelated to this PR, but is it easy to to change threshold alert type to threshold rule type?

[mgiota]

@maryam-saeidi Code review LGTM! I need to test it locally as well.

What do BE and FE mean?

[mgiota]

I created a custom threshold rule and everything works as expected!

[maryam-saeidi]

What do BE and FE mean?

@mgiota I mean, in the rule flyout (front-end), we already removed the warning threshold, so it is only possible to set the alert threshold. By back-end, I mean the rule execution logic in the server folder.