Fix PopConfirm crash when Placement is PlacementMode.Custom

[Copilot]

Setting PopConfirm.Placement to PlacementMode.Custom crashed the application, since Custom requires a positioning delegate that PopConfirm does not supply.

Changes

• PopConfirm.PlacementProperty: added a coerce callback that maps PlacementMode.Custom to PlacementMode.Center, leaving all other values untouched.

public static readonly StyledProperty<PlacementMode> PlacementProperty =
    Popup.PlacementProperty.AddOwner<PopConfirm>(
        new StyledPropertyMetadata<PlacementMode>(
            coerce: static (_, value) => value is PlacementMode.Custom ? PlacementMode.Center : value));

• Tests: added headless tests covering that Custom coerces to Center and non-Custom values (e.g. Top) are preserved.

Note

The test suite could not be executed in this environment because the Avalonia nightly NuGet feed is unreachable (NU1301), blocking package restore.

• Fixes PopConfirm 的 Placement = PlacementMode.Custom 时会闪退 #1071

[github-actions]

Test Results

0 tests  ±0   0 ✅ ±0   0s ⏱️ ±0s
0 suites ±0   0 💤 ±0 
0 files   ±0   0 ❌ ±0 

Results for commit f82c4f0. ± Comparison against base commit b910a09.

♻️ This comment has been updated with latest results.

[github-actions]

Package	Line Rate	Branch Rate	Complexity	Health
Ursa	47%	35%	11142	✔
Ursa.Themes.Semi	62%	66%	2887	✔
Summary	55% (18665 / 34043)	37% (3434 / 9250)	14029	✔