Always push down TopN when N is less than a specific variable

[time-and-fate]

Enhancement

Currently, in the planner, the choice between push down TopN or not is cost-based. But sometimes the cost calculation is not accurate enough to make the correct choice. When we choose not to push down TopN when it should, the SQL execution will become very slow sometimes. Considering the risk and benefit, always choose to push down the TopN may be a better strategy.

[time-and-fate]

/sig planner

[qw4990]

Please see #26550 for more details