Feat/9243 dogfood eslint plugin eui

[ragini-pandey]

Summary

This PR implements issue #9243 by dogfooding the @elastic/eslint-plugin-eui in the packages/eui package.

Changes made:

1. Added @elastic/eslint-plugin-eui as a workspace devDependency in packages/eui/package.json
2. Extended the plugin's recommended config and added it to the plugins list in packages/eui/.eslintrc.js
3. Fixed a critical bug in the no-css-color rule where it would crash when encountering arrow functions that return non-object expressions (e.g., css={() => undefined}). Added proper null/type checking before calling .filter() on properties.

The plugin now successfully runs on the EUI codebase, reporting 786 warnings across various accessibility and code quality rules.

Why are we making this change?

Closes #9243

By using our own ESLint plugin in the EUI package, we can:

• Catch the same types of issues we warn consumers about in our own code
• Ensure the plugin rules work correctly on a large real-world codebase
• Lead by example and improve code quality consistency

Screenshots

Not applicable - this is a development tooling change

Impact to users

No impact to end users. This is an internal development change that only affects contributors to the EUI repository. The ESLint plugin was already published and available to consumers; this PR simply enables it within the EUI package itself for improved code quality during development.

QA

General checklist

• Browser QA - Not applicable (development tooling only)
• Docs site QA - Not applicable (no user-facing changes)
• Code quality checklist
  • Tested that ESLint runs successfully with the plugin enabled
  • Verified that the plugin correctly reports warnings across the codebase
  • Fixed bug preventing ESLint from completing successfully
• Release checklist
  • A changelog entry exists and is marked appropriately
  • No breaking changes
  • No cross-repo dependencies

[github-actions]

👋 Since this is a community submitted pull request, a Buildkite build has not been started automatically. Would an Elastic organization member please verify the contents of this pull request and kick off a build manually?

[weronikaolejniczak]

buildkite test this

[weronikaolejniczak]

I ran the ESLint plugin tests locally and they all passed. I ran the linter and it's reporting only warnings, and in line with what we have in our ESLint plugin. The code looks correct. Thank you for finding and addressing that critical issue in no_css_color. 🟢 Great contribution, @ragini-pandey! 🙌🏻

[weronikaolejniczak]

buildkite test this

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 86769e7
• Documentation website
• Storybook

History

• 💚 Build #2606 succeeded 0adf6fc

[elasticmachine]

💚 Build Succeeded

• Buildkite Build
• Commit: 86769e7

History

• 💚 Build #5730 succeeded 0adf6fc

[ragini-pandey]

I ran the ESLint plugin tests locally and they all passed. I ran the linter and it's reporting only warnings, and in line with what we have in our ESLint plugin. The code looks correct. Thank you for finding and addressing that critical issue in no_css_color. 🟢 Great contribution, @ragini-pandey! 🙌🏻

Thank you for the review @weronikaolejniczak!
Buildkite is also green too!