Adopt ESLint 10 / flat config

[SuZhou-Joe]

Description

Migrates the plugin to ESLint 10 flat config. Adds eslint.config.js that spreads @elastic/eslint-config-kibana plus the EUI config (preserving require-license-header off), and removes the legacy .eslintrc. Fixes the lint script in package.json, which used the -c/--ignore-path flags that ESLint 10 removed. Removes the Prettier 2 pin, since Prettier is inherited from OpenSearch Dashboards core.

This is a config-only change; no source files were reformatted.

Issues Resolved

Closes #1485

Check List

• Commits are signed per the DCO using --signoff

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[riysaxen-amzn]

Thanks for this @SuZhou-Joe — clean migration overall.

One question: with the old lint script using --ignore-path .gitignore, is there an equivalent ignores array needed in the flat config? Or does ../../scripts/eslint (or the shared OSD config) handle ignoring node_modules, build/, etc. automatically?

Also, 3 of the 17 checks appear to be failing — are those pre-existing/unrelated?

Otherwise LGTM. Config-only change, idiomatic flat config pattern, and the Prettier pin removal makes sense since it's inherited from core.

[SuZhou-Joe]

with the old lint script using --ignore-path .gitignore, is there an equivalent ignores array needed in the flat config

Thanks for the sharp eyes, I updated the config a little bit to reflect previous ignore behavior.

[SuZhou-Joe]

E2E Cypress tests / Run document_level_monitor Cypress tests (pull_request)

It is an existing failure and can be found in other PRs.

Install Dashboards with Plugin via Binary / Run binary installation (ubuntu-latest) (pull_request)

Failed due to not pinned to a commit SHA

Unit tests workflow / Run unit tests (pull_request)

Failed due to yarn not installed, the error can be found in other PRs.