Bugfix: only round to the nearest cell for selection (#18486)

## Summary of the Pull Request
Fixes a bug where VT mouse mode would round to the nearest cell when
clicking the mouse button.
The fix is to round to the nearest cell only when we're selecting text.
The other scenarios affected are:
- clicking on a hyperlink
- vt mouse mode
- where the context menu is anchored

Really the most notable ones were the first two. So now, we use the
position of the cell we clicked on. We only round for selection.

## References and Relevant Issues
Follow-up to #18106

## Detailed Description of the Pull Request / Additional comments

## Validation Steps Performed
Opened Midnight Commander in Ubuntu and clicked between the two panes.
- Before: threshold was too early to switch between panes
- After: threshold is clearly separated between the outline of the two
panes

---------

Co-authored-by: Dustin L. Howett <duhowett@microsoft.com>

src/cascadia/TerminalControl/ControlInteractivity.cpp

@@ -241,7 +241,8 @@ namespace winrt::Microsoft::Terminal::Control::implementation
                                               const ::Microsoft::Terminal::Core::ControlKeyStates modifiers,
                                               const Core::Point pixelPosition)
     {
-        const auto terminalPosition = _getTerminalPosition(til::point{ pixelPosition }, true);
+        // Un-rounded coordinates; we only round when selecting text
+        const auto terminalPosition = _getTerminalPosition(til::point{ pixelPosition }, false);
 
         const auto altEnabled = modifiers.IsAltPressed();
         const auto shiftEnabled = modifiers.IsShiftPressed();
@@ -285,7 +286,8 @@ namespace winrt::Microsoft::Terminal::Control::implementation
             }
             const auto isOnOriginalPosition = _lastMouseClickPosNoSelection == pixelPosition;
 
-            _core->LeftClickOnTerminal(terminalPosition,
+            // Rounded coordinates for text selection
+            _core->LeftClickOnTerminal(_getTerminalPosition(til::point{ pixelPosition }, true),
                                        multiClickMapper,
                                        altEnabled,
                                        shiftEnabled,