fix: correct midpoint calculation in convert_point_to_coordinates

[kuishou68]

Closes #242

Problem

In codes/ui_tars/action_parser.py, the convert_point_to_coordinates function incorrectly computes the center of a point using x1+x1 and y1+y1 instead of x1+x2 and y1+y2. This copy-paste error causes the function to always return the top-left corner (x1, y1) of the bounding box instead of its actual center.

# BUG (before)
x = (x1 + x1) // 2
y = (y1 + y1) // 2

# FIX (after)
x = (x1 + x2) // 2
y = (y1 + y2) // 2

Impact

All point-to-coordinate conversions silently return the wrong location, leading to incorrect click/interaction targets in the UI agent.