Fix CJK IME key events on Windows

[happykawayigt]

Link to issue or discussion

Fixes #6667.

Summary

• preserve valid Unicode characters from Windows console events whose virtual-key code is zero
• continue filtering pure modifier events that carry no Unicode character
• add Windows-specific regression coverage for both paths

Testing

• pytest tests/test_win32.py tests/test_xterm_parser.py -q (69 passed, 1 xfailed)
• black --check src/textual/drivers/win32.py tests/test_win32.py
• git diff --check