Should we use the new `expectedVisitedNodes` estimator to speed up filtered vector search?

### Description

@benwtrent 's PR at #14836 introduced an estimator for the number of visited nodes based on k and the number of vectors called `expectedVisitedNodes(int k, int size)`.

Currently, filtered vector search works roughly this way:
1. compute a per-leaf k value based on the global k value
2. if (filterCost <= perLeafK) do an exact search
3. else run an approximate search, stop after visiting filterCost+1 nodes
4. if the approximate search stopped early (and is thus incomplete), run an exact search

Could/should we update step 2 above to do an exact search if (filterCost <= expectedVisitedNodes(perLeafK, size))? This should help save approximate searches that are almost certainly going to be stopped early and ignored anyway?

Start by reading PR #14836's expectedVisitedNodes(int k, int size) estimator and locate the filtered vector-search code implementing the four described steps. Compare the current per-leaf exact-search decision with the estimator, then verify that any change preserves filtered-search correctness and avoids approximate work when the estimate indicates it will be stopped early.

Written by the indexing model from the issue text.