Coordinate flip in Picking.js causes out-of-bounds access when y=0

### What happened?

Hello ,
In the `pickPositionWorldCoordinates` function at line 632:
https://github.com/CesiumGS/cesium/blob/7355e1eba2f0732a694c3e5f03960caa982ff7e1/packages/engine/Source/Scene/Picking.js#L632

There is a Y-coordinate flip:
` drawingBufferPosition.y = scene.drawingBufferHeight - drawingBufferPosition.y;`

However, this creates an off-by-one error. When drawingBufferPosition.y = 0, the result becomes scene.drawingBufferHeight, which is out of bounds since valid pixel coordinates should range from [0, scene.drawingBufferHeight - 1].
For example, with a drawing buffer height of 270:

Input: y = 0 → Output: y = 270 (invalid - should be 269)
Input: y = 269 → Output: y = 1 (valid)

Should this be corrected to:
` drawingBufferPosition.y = scene.drawingBufferHeight - drawingBufferPosition.y - 1 `

This would ensure the coordinate stays within valid bounds [0, height-1] after the flip.
Is this intentional behavior, or should it be fixed?

Start in packages/engine/Source/Scene/Picking.js at line 632, within pickPositionWorldCoordinates. Check the coordinate flip with y=0 and y=269 against the stated drawing-buffer bounds. Done means the flip no longer produces an out-of-bounds y value while preserving valid coordinate behavior.

Written by the indexing model from the issue text.