Only check optional ActionCable Redis instance if necessary

What does this MR do and why?

!214008 (merged) aadded Gitlab::Redis::ActionCable to ALL_CLASSES, which caused the Admin Dashboard to fail with a Redis connection error when redis.action_cable.yml is not configured.

On an Omnibus instance, it's possible to configure Redis-specific instances and disable the default Redis server:

redis['enable'] = false
gitlab_rails['redis_cache_instance'] = 'redis://localhost'
gitlab_rails['redis_queues_instance'] = 'redis://localhost'
gitlab_rails['redis_actioncable_instance'] = 'redis://localhost'
gitlab_rails['redis_shared_state_instance'] = 'redis://localhost'

Prior to GitLab 18.7.0, this configuration worked fine. However, after !214008 (merged) gitlab_rails['redis_host'] must be configured or the /admin page would throw a 500 error because the default Redis server was not available.

To avoid this error, introduce an active? method to determine if a Redis instance is configured and available:

• Wrapper base class: Returns true by default (all Redis instances are active)
• ActionCable: Returns false if redis.action_cable.yml doesn't exist
• Dashboard & Health Checks: Only include active Redis instances when collecting versions or creating health checks

This allows ActionCable to be optional without breaking deployments where it's not configured.

Relates to https://gitlab.com/gitlab-com/request-for-help/-/work_items/4037

How to set up and validate locally

On an Omnibus instance:

1. Run Redis in one window via /opt/gitlab/embedded/bin/redis-server.
2. Add this to /etc/gitlab/gitlab.rb:

redis['enable'] = false
gitlab_rails['redis_cache_instance'] = 'redis://localhost'
gitlab_rails['redis_queues_instance'] = 'redis://localhost'
gitlab_rails['redis_actioncable_instance'] = 'redis://localhost'
gitlab_rails['redis_shared_state_instance'] = 'redis://localhost'

3. Run gitlab-ctl reconfigure.
4. Visit the /admin page and get a 500 error.
5. Also run curl "http://localhost:8080/-/readiness?all=1" | jq and check that this error is present:

  "action_cable_check": [
    {
      "status": "failed",
      "message": "unexpected Action_cable check result: No such file or directory - connect(2) for /var/opt/gitlab/redis/redis.socket (unix:///var/opt/gitlab/redis/redis.socket)"
    }

As root, apply this patch:

curl -o /tmp/config.patch https://gitlab.com/gitlab-org/gitlab/-/merge_requests/219019.diff
cd /opt/gitlab/embedded/service/gitlab-rails
patch -p1 < /tmp/config.patch
gitlab-ctl restart puma

Repeat steps 4 and 5 and verify no error.

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.