Fix moving selection past scroll area (#13318)

## Summary of the Pull Request
Introduced in #10824, this fixes a bug where you could use keyboard selection to move below the scroll area. Instead, we now clamp movement to the mutable viewport (aka the scrollable area). Specifically, we clamp to the corners (i.e. 0,0 or bottom right cell of scroll area).

## Validation Steps Performed
✅ (no output) try to move past bottom of viewport
✅ (with output, at bottom of scroll area) try to move past viewport
✅ (with output, NOT at bottom of scroll area) try to move past viewport
✅ try to move past top of viewport

src/cascadia/TerminalCore/TerminalSelection.cpp

@@ -376,7 +376,7 @@ void Terminal::UpdateSelection(SelectionDirection direction, SelectionExpansion
     // signifying that the other endpoint is the one we want to move.
     auto targetPos{ MovingEnd() ? _selection->end : _selection->start };
 
-    // 2. Perform the movement
+    // 2.A) Perform the movement
     switch (mode)
     {
     case SelectionExpansion::Char:
@@ -393,6 +393,9 @@ void Terminal::UpdateSelection(SelectionDirection direction, SelectionExpansion
         break;
     }
 
+    // 2.B) Clamp the movement to the mutable viewport
+    targetPos = std::min(targetPos, _GetMutableViewport().BottomRightInclusive());
+
     // 3. Actually modify the selection
     if (_markMode && !mods.IsShiftPressed())
     {
@@ -410,17 +413,17 @@ void Terminal::UpdateSelection(SelectionDirection direction, SelectionExpansion
     }
 
     // 4. Scroll (if necessary)
-    if (const auto viewport = _GetVisibleViewport(); !viewport.IsInBounds(targetPos))
+    if (const auto visibleViewport = _GetVisibleViewport(); !visibleViewport.IsInBounds(targetPos))
     {
-        if (const auto amtAboveView = viewport.Top() - targetPos.Y; amtAboveView > 0)
+        if (const auto amtAboveView = visibleViewport.Top() - targetPos.Y; amtAboveView > 0)
         {
             // anchor is above visible viewport, scroll by that amount
             _scrollOffset += amtAboveView;
         }
         else
         {
             // anchor is below visible viewport, scroll by that amount
-            const auto amtBelowView = targetPos.Y - viewport.BottomInclusive();
+            const auto amtBelowView = targetPos.Y - visibleViewport.BottomInclusive();
             _scrollOffset -= amtBelowView;
         }
         _NotifyScrollEvent();