WIP: Web terminal for CI builds, done right

What does this MR do?

Adds support for web terminals in CI builds using gitlab-runner!857 (closed). This implementation is executor-agnostic and avoids the "dual brain" problem of the earlier approach in !17281 (closed).

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes #25990 (closed)

• Depends on gitlab-runner!857 (closed) and gitlab-workhorse!234 (merged)
• Old issue about adding web terminals for environments: #22864 (closed)