Fix redis tasking unit tests - #8014

Assisted by: cursor-grok-4.6

Switch the fixture in the test to use redisdb fixture instead of trying to talk to the redis instance on localhost (which would fail on the CI with our rewritten host and thus silently skip the test).

Also updated the safe_release_task_locks to do nothing if the redis connection isn't there.

📜 Checklist

• Commits are cleanly separated with meaningful messages (simple features and bug fixes should be squashed to one commit)
• A changelog entry or entries has been added for any significant changes
• Follows the Pulp policy on AI Usage
• (For new features) - User documentation and test coverage has been added

See: Pull Request Walkthrough