exp: No overlaps when adding multiple table nodes at once (#4072)

## Summary
Fixes an issue where multiple tables added to the query builder would
overlap instead of being positioned side-by-side. The placement logic
now properly spaces new nodes by using the first node's API-determined
position and placing subsequent nodes relative to it.

  ## Changes
- Simplified node placement logic in `ensureNodeLayouts` to track the
last placed node
- First node uses `NodeGraphApi.findPlacementForNode()` for optimal
placement
- Subsequent nodes are positioned horizontally offset from the previous
node
- Early return when `nodeGraphApi` is null to avoid unnecessary
processing

  ## Notes
The previous logic only worked correctly when the API was unavailable
(fallback case), but when the API was present, all nodes would receive
the same initial placement from `findPlacementForNode()`, causing
overlaps.