[kbn-eslint-plugin-eslint] add scout_api_test_encourage_api_client rule

[steliosmavro]

Summary

Adds a new ESLint rule to encourage the use of the apiClient fixture in API tests within Scout. The rule emits a warning if an API test does not use apiClient, guiding test authors to follow our best practice of using apiClient for endpoint validation rather than apiServices or kbnClient in the test body.

Why

• Ensures API tests are explicit about which client is used for endpoint validation.
• Reduces confusion and potential privilege issues caused by using apiServices or kbnClient directly in test bodies.

What changed

• Introduced a new ESLint rule that checks for apiClient usage in API test blocks.
• The rule emits a warning if apiClient is not used, but can be overridden with a comment in edge cases.

How to use

• Write API tests as usual, ensuring that apiClient is used for the main endpoint validation.
• If the rule triggers in a valid edge case, suppress the warning with an ESLint comment and add a justification.

[cla-checker-service]

❌ Author of the following commits did not sign a Contributor Agreement:
6f780c7

Please, read and sign the above mentioned agreement if you want to contribute to this project

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 6f780c7

Metrics [docs]

Unknown metric groups

ESLint disabled in files

id	before	after	diff
@kbn/eslint-plugin-eslint	0	2	+2

ESLint disabled line counts

id	before	after	diff
@kbn/eslint-plugin-eslint	0	2	+2

Total ESLint disabled count

id	before	after	diff
@kbn/eslint-plugin-eslint	0	4	+4