fix: convert_point_to_coordinates computes x and y from x1+x1 instead of x1+x2

## Bug

In `codes/ui_tars/action_parser.py`, the `convert_point_to_coordinates` function contains a copy-paste error:

```python
x = (x1 + x1) // 2 # BUG: should use x2
y = (y1 + y1) // 2 # BUG: should use y2
```

Both `x` and `y` are computed using only the first coordinate repeated (`x1+x1` and `y1+y1`), instead of averaging the two coordinates (`x1+x2` and `y1+y2`). This means the midpoint is always equal to `x1` and `y1`, ignoring `x2` and `y2` entirely — so the function always returns the top-left corner of the bounding box instead of its center.

## Fix

Change:
```python
x = (x1 + x1) // 2
y = (y1 + y1) // 2
```
To:
```python
x = (x1 + x2) // 2
y = (y1 + y2) // 2
```

Open codes/ui_tars/action_parser.py and inspect convert_point_to_coordinates. Verify that the midpoint uses both x coordinates and both y coordinates as described in the issue. Done means the function averages x1 with x2 and y1 with y2 instead of repeating the first coordinate.

Written by the indexing model from the issue text.