Feature/COMMS-1025: Introduce the work package labels feature flag

[akabiru]

Ticket: https://community.openproject.org/wp/COMMS-1025

Adds the work_package_labels feature flag so the labels work (API in #25378, journaling in #25320, the administration page and the work package UI) can land in independent PRs behind one switch and stay hidden until we enable it for production. Nothing is gated yet; the stacked PRs #25401 and #25394 are the first consumers.

[github-actions]

Warning

Flaky specs

• rspec ./modules/gantt/spec/features/timeline/timeline_dates_spec.rb[1:2:1]
• rspec ./modules/meeting/spec/features/presentation/meeting_presentation_mode_spec.rb[1:1]
• rspec ./modules/wikis/spec/features/admin/internal_provider_spec.rb[1:1]
• rspec ./spec/features/notifications/navigation_spec.rb[1:1:1]

🤖 Ask Copilot to investigate

Copy the prompt below into a new comment on this PR to delegate the investigation to GitHub Copilot. It will look into the flakiness and open a separate pull request with you as reviewer.

@copilot The following spec(s) are flaky in CI (first seen on PR #25393, linked for reference only):

- `rspec ./modules/gantt/spec/features/timeline/timeline_dates_spec.rb[1:2:1]`
- `rspec ./modules/meeting/spec/features/presentation/meeting_presentation_mode_spec.rb[1:1]`
- `rspec ./modules/wikis/spec/features/admin/internal_provider_spec.rb[1:1]`
- `rspec ./spec/features/notifications/navigation_spec.rb[1:1:1]`

Treat this as a standalone task, unrelated to PR #25393. Create a new branch from origin/dev and open a new pull request targeting dev — do not stack it on PR #25393 or reuse that branch.

Follow the playbook in docs/development/testing/handling-flaky-tests/README.md to find the root cause and fix the underlying race — do not skip, delete, or weaken the spec to make it pass; disabling is a last resort per the playbook, and only with a bug ticket. Verify the fix by running the spec(s) repeatedly (e.g. `script/bulk_run_rspec --run-count 10`).

If you cannot reproduce the flake or are not confident in a fix after reasonable investigation, do not fabricate a change or skip the spec to force CI green. Instead, leave the pull request in draft and document what you tried, the suspected cause, and any leads in its description, then assign @akabiru to take over.

Once the fix is verified, title the PR after the spec(s) it fixes, and use the PR description to explain the root cause, how the change resolves it, and the before/after results. Label the PR `flaky-spec`, assign @akabiru, and request a review from @akabiru.
On every commit, set @akabiru as the sole co-author with a `Co-authored-by:` trailer (use their GitHub no-reply email so it links to their account), so it is traceable who dispatched the fix.

[github-actions]

Caution

The provided work package version does not match the core version

Details:

• Work package URL: https://community.openproject.org/wp/COMMS-1025
• Work package version: not set
• Core version: 18.0.0

Please make sure that:

• The work package version OR your pull request target branch is correct

[brunopagno]

Let's go!