Fix TextArea scrollbar initial wrap flicker (#5103)

[glaziermag]

Fix TextArea scrollbar initial wrap flicker (#5103)

This commit forces an internal geometry double-pass exclusively during _rewrap_and_refresh_virtual_size if the wrap calculation triggers an appearance/disappearance of the scrollbar dynamically. This mathematically sets the width before flushing the virtual DOM to the visual layer, preventing the flicker loop reported in #5103.

The Epistemic Before vs After Logs

Input Command:

$ python3 scratch/test_layout.py

Before Logs:
Notice how the layout unpredictably triggers events during resize pushing to an async queue and bouncing between 15 and 13.

=== HEADLESS RENDER OUTCOME ===
REWRAP: width=0, v_scroll=False, size=Size(width=0, height=0), scroll_region=Size(width=0, height=0)
REWRAP: width=15, v_scroll=False, size=Size(width=16, height=3), scroll_region=Size(width=16, height=3)
  File "/Users/gabe/textual/src/textual/widgets/_text_area.py", line 1085, in _on_resize
    self._rewrap_and_refresh_virtual_size()

REWRAP: width=13, v_scroll=True, size=Size(width=16, height=3), scroll_region=Size(width=14, height=3)
  File "/Users/gabe/textual/src/textual/reactive.py", line 355, in _set
  File "/Users/gabe/textual/src/textual/widgets/_text_area.py", line 861, in _watch_show_vertical_scrollbar
    self._rewrap_and_refresh_virtual_size()

After Logs:
The geometry natively stabilizes in memory perfectly inside the bounding box before being rendered.

=== HEADLESS RENDER OUTCOME ===
Container Size: Size(width=16, height=3)
Scrollable Content Region Size: Size(width=14, height=3)
Computed wrap_width: 13
Scrollables: vertical=True, horizontal=False
Wrapped lines total: 12
  Line 0: length=13 'XXXXXXXXXXXXX'

302 Pytest assertions passed locally in 33.49s ensuring no downstream caching regressions.