Add CI_ENVIRONMENT_ID predefined variable

• Please check this box if this contribution uses AI-generated content (including content generated by GitLab Duo features) as outlined in the GitLab DCO & CLA. As a benefit of being a GitLab Community Contributor, you receive complimentary access to GitLab Duo.

What does this MR do and why?

Add CI_ENVIRONMENT_ID predefined variable

References

Related to #515854 (comment 2658297133)

Variables like CI_ENVIRONMENT_SLUG are not available for options like job:environment:kubernetes:namespace. To use them for these fields, you need to edit the environment via API, but that requires the environment id which is not yet available. Therefore I'm adding it so that jobs can easily know which environment they have to edit instead of listing all environments of the project and then finding the correct one based on CI_ENVIRONMENT_NAME.

Screenshots or screen recordings

Before	After

How to set up and validate locally

MR acceptance checklist

Evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.