[18.8] Unsubscribe from pipeline status updates when pipeline ID changes

What does this MR do and why?

This backports !221586 (merged) to 18-8-stable-ee.

When the pipeline ID changes during polling or when the component is destroyed, properly unsubscribe from the previous pipeline's status updates. This prevents accumulating multiple active subscriptions over time, which could result in hundreds of subscription requests if a tab is left open for an extended period.

The subscribeToMore() method returns an object with an unsubscribe() method, so we now store the subscription and call unsubscribe() when switching to a new pipeline or on component destruction.

Relates to #587107

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