[Security Solution][RAC] Fixes Threshold Rule functionality for post-rule-migration 8.0

[madirey]

Summary

Threshold rules were broken after merging #112113 because the UI was referencing the signal.* fields, whereas new alerts are generated using the AAD schema. Additionally, threshold alerts were being generated incorrectly, placing threshold.result at the top level of the document instead of under kibana.alert.threshold_result where it belongs.

This PR:

• Adjusts post-migration Threshold Rules to generate correct documents (using kibana.alert.threshold_result)
• Reenables Threshold Rule tests
• Removes unused code (nonEcsData, getFilterAndRuleBounds)
• Changes threshold field references in UI to use new AAD fields and fall back to legacy signal.* fields

Checklist

Delete any items that are not applicable to this PR.

• Any text added follows EUI's writing guidelines, uses sentence case text and includes i18n support
• Documentation was added for features that require explanation or tutorials
• Unit or functional tests were updated or added to match the most common scenarios
• Any UI touched in this PR is usable by keyboard only (learn more about keyboard accessibility)
• Any UI touched in this PR does not create any new axe failures (run axe in browser: FF, Chrome)
• If a plugin configuration key changed, check if it needs to be allowlisted in the cloud and added to the docker list
• This renders correctly on smaller devices using a responsive layout. (You can test this in your browser)
• This was checked for cross-browser compatibility

Risk Matrix

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Before closing this PR, invite QA, stakeholders, and other developers to identify risks that should be tested prior to the change/feature release.

When forming the risk matrix, consider some of the following examples and how they may potentially impact the change:

Risk	Probability	Severity	Mitigation/Notes
Multiple Spaces—unexpected behavior in non-default Kibana Space.	Low	High	Integration tests will verify that all features are still supported in non-default Kibana Space and when user switches between spaces.
Multiple nodes—Elasticsearch polling might have race conditions when multiple Kibana nodes are polling for the same tasks.	High	Low	Tasks are idempotent, so executing them multiple times will not result in logical error, but will degrade performance. To test for this case we add plenty of unit tests around this logic and document manual testing procedure.
Code should gracefully handle cases when feature X or plugin Y are disabled.	Medium	High	Unit tests will verify that any feature flag or plugin combination still results in our service operational.
See more potential risk examples

For maintainers

• This was checked for breaking API changes and was labeled appropriately

[madirey]

@elasticmachine merge upstream

[kibanamachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 640439b
• Storybooks Preview

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
securitySolution	4.5MB	4.5MB	-90.0B

History

• 💛 Build #4552 was flaky b0f9b8c
• 💔 Build #4503 failed 6c9199e
• 💔 Build #4449 failed dd0758f

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

[elasticmachine]

Pinging @elastic/security-solution (Team: SecuritySolution)

[marshallmain]

Code changes LGTM! Can you add a couple more sentences to the PR to add some details about what's being fixed here and how it behaves before and after the fix?

[kibanamachine]

💚 Backport successful

Status	Branch	Result
✅	8.0

This backport PR will be merged automatically after passing CI.

[madirey]

Thanks @marshallmain ... done!