Investigate window-function alias loss in planner snapshot expectations

### Motivation
`pinot-query-planner/src/test/resources/queries/WindowFunctionPlans.json` repeats the note `TODO: Look into why aliases are getting ignored in the final plan` across multiple cases.

That suggests either a genuine alias-propagation bug in the planner or stale expected outputs in the snapshot file.

### Scope
- Determine whether aliases should survive to the final explain plan for these window queries.
- Fix planner behavior or refresh the plan snapshots.
- Remove the repeated TODO notes once expectations are stable.

### Notes
Observed on `upstream/master` on April 15, 2026.

Start with pinot-query-planner/src/test/resources/queries/WindowFunctionPlans.json and review the repeated alias TODOs for the window-query cases. Determine whether aliases should appear in the final explain plans, then inspect the planner behavior or snapshot expectations accordingly. Done means the behavior or snapshots are stable and all repeated TODO notes are removed.

Written by the indexing model from the issue text.