[18.7] Only check optional ActionCable Redis instance if necessary

What does this MR do and why?

This backports !219019 (merged) to 18-7-stable-ee.

!214008 (merged) added 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.

References

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

How to set up and validate locally

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• Ensure the e2e:test-on-omnibus-ee job has succeeded, or if it has failed, investigate the failures. If you determine the failures are unrelated, you may proceed. If you need assistance investigating, reach out to a Software Engineer in Test in #s_developer_experience.

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