IGDataChart: demonstrate new MarkerSize in existing Marker Types sample

[Copilot]

This updates the WPF DataChart marker samples to explicitly demonstrate the new MarkerSize API introduced in 26.1.
Instead of adding a new sample, the existing Marker Types sample now shows both marker shape and marker size control in one place.

• Sample update: Display/Markers/MarkerTypes

  • Wired SplineSeries.MarkerSize to a UI slider for live resizing.
  • Added compact inline controls in the sample header:
    • Marker Size label
    • size slider (6..40, default 20, snap-to-tick)
    • current numeric value display

• Why this sample

  • MarkerTypes already demonstrates marker shape selection; adding size control keeps marker customization scenarios co-located and easy to discover.

• Implementation snippet

  <ig:SplineSeries
      MarkerType="{Binding ElementName=cmbMarkerType, Path=SelectedItem.Tag, Converter={StaticResource MarkerTypeConverter}}"
      MarkerSize="{Binding ElementName=markerSizeSlider, Path=Value}"
      ... />
  
  <TextBlock Text="Marker Size" ... />
  <Slider x:Name="markerSizeSlider"
          Minimum="6" Maximum="40" Value="20"
          TickFrequency="2" IsSnapToTickEnabled="True" ... />
  <TextBlock Text="{Binding ElementName=markerSizeSlider, Path=Value, StringFormat={}{0:0}}" ... />

• Screenshot
  

[IGvaleries]

this was done in the wrong branch, I tried changing the base branch but it seems some stuff got added I think it may need to be redone.

For this repo, we update vnext as master is what our customers use (hence it is the default branch for them), so until we actually release the VR we should update vnext and then after we release the product we need to merge vnext into master