Feature/column autofit on double click

[HannahVernon]

Auto-fit column width on double-click of resize grip

What

Adds the standard "double-click column separator to auto-fit" behavior to DataGridColumnHeader, matching WPF's DataGrid.

Before: Double-clicking the resize grip does nothing (or triggers sort on the adjacent header).
After: Double-clicking the resize grip auto-sizes the column to fit its header and visible cell content.

How

Single addition to DataGridColumnHeader.cs (~60 lines, no API surface changes):

1. New DoubleTapped handler in the constructor
2. Uses the existing DATAGRIDCOLUMNHEADER_resizeRegionWidth constant for grip detection — right edge fits that column, left edge fits the column to the left
3. Respects CanResizeColumn() (both grid-level and column-level settings)
4. Temporarily sets Auto sizing via SetWidthInternalNoCallback to trigger content measurement, then locks to a fixed pixel width via Dispatcher.Post for predictable subsequent drag behavior
5. Respects ActualMinWidth / ActualMaxWidth constraints

Why

This is standard behavior in WPF DataGrid, WinForms DataGridView, Excel, and virtually every desktop grid control. Users coming from any of these environments instinctively double-click column separators to auto-fit. I searched the Avalonia and Avalonia.Controls.DataGrid issue trackers and found no prior feature request.

We implemented an external workaround for our project, but it has limitations (string-based column matching since OwningColumn is internal, estimated grip zone, layout timing hacks). This native implementation avoids all of those by using internal APIs directly.

Testing

• All 20 existing unit tests pass
• Manually verified in our Avalonia 11.x app via the external workaround using the same approach

[HannahVernon]

This addresses issue #225

[kdurane]

I added you double tap method to my fork, as I agree, its a great feature to have, however, I wasn't able to make it work. I did make it work by asking the datagrid to do it itself, by using OwningColumn.SetWidthInternalNoCallback(DataGridLength.Auto);
columnToAutoFit.OwningGrid?.InvalidateMeasure(); (at the end of your method) instead of AutoFitColumnWidth method