Mouse selection under tmux is broken

[sgmunn]

from Jason.

1. Make sure you have set -g mouse on inside ~/.tmux.conf (my full tmux config is here - https://github.com/nosami/configs/blob/master/.tmux.conf
2. Launch tmux session
3. Try and select text

same on XtermSharp.

[gi11es]

A fix has been submitted in #519.

The issue is that mouseDragged used sendMotionEvent() as its guard, which only matches .anyEvent mode. tmux uses .buttonEventTracking (CSI ? 1002 h), which also needs drag events forwarded for pane resizing and mouse selection. The fix changes the guard to sendButtonTracking(), which covers both modes.

The PR includes 14 tests covering the MouseMode predicate behavior and integration tests verifying SGR motion event output.