frontend: fix Redpanda Connect list pagination issues

[malinskibeniamin]

Fix infinite loop in Redpanda Connect Pipeline list page pagination

Root Cause: PipelineListPage stored <PipelineListPageContent /> in a JSX variable that was recreated on every parent render, causing the child component to unmount/remount constantly and trigger infinite loops when interacting with pagination.

Fix: Converted inline JSX variable to a proper RedpandaConnectContent component to maintain stable component identity across renders.

Additional improvements:

• Fixed useInfiniteQueryWithAllPages hook by removing fetchNextPage from useEffect deps (prevents infinite loops in other features using this hook)
• Stabilized useListPipelinesQuery return values with useMemo to prevent unnecessary re-renders
• Increased default page size from 10 to 20 rows