Sub-issue 5 of #22034.
Right-click on the composer always pasted from the clipboard, even when
the user had highlighted text — diverging from terminal-native behavior
(xterm/iTerm/gnome-terminal) where right-click copies an active selection
and only pastes when nothing is selected.
Extract a small pure helper, decideRightClickAction(value, range), and
route the existing onMouseDown right-click branch through it. Selection
present and non-empty -> writeClipboardText(slice). Otherwise fall back
to the existing emitPaste path.
a2920b1762