fix(ui): drawers close on any click outside the rail — TEST was the missing third

Creator: 'when the test slide-out is active, then any click off the div should
close the div — this behaviour applies to all tabs, not just TEST.'

MainWindow.Window_PreviewMouseLeftButtonDown already closed the Stream Settings
and YPP drawers on any click outside TextPullOutHost, but its close list skipped
TestSession — so the TEST drawer never dismissed on an outside click.

Fix: run TestSession.CloseDrawerCommand in the same outside-click branch.
All three drawers share the rail host, so the containment check is unchanged.

Good Dog: ONE integration test — the existing TestStream window section now
raises a window-root PreviewMouseLeftButtonDown (source = window => outside the
rail; deterministic, no OS mouse) and asserts the TEST and Stream Settings
drawers both close. Gate: clean build 0 warnings, full vstest 318/318.

Controls/PreviewPane.xaml

@@ -418,7 +418,7 @@
                          edge; each toggles its own drawer (the same animated-width pattern).
                          Only one drawer is open at a time — IsDrawerOpen exclusivity is
                          VM-enforced (opening one collapses the other). Any click outside
-                         both collapses whichever is open (MainWindow code-behind). -->
+                         the rail collapses whichever is open (MainWindow code-behind). -->
                     <Grid Grid.Row="0">
                         <StackPanel x:Name="TextPullOutHost" Orientation="Horizontal"
                                     HorizontalAlignment="Right" VerticalAlignment="Stretch">