Position DatePicker and TimePicker popups below input fields

[Keboo]

Date and time picker popups were using a custom placement callback that could obscure the selected value. This change aligns picker behavior with the expected default by opening below the input field in normal layouts.

What changed

• Switched DatePicker and TimePicker template popup placement from Custom to Bottom.
• Removed the now-unused CustomPopupPlacementCallbackHelper.
• Added UI coverage for both controls to assert the popup opens below the text box by default.

Notes for reviewers

• This intentionally relies on normal WPF popup fallback behavior, so placement can still flip when there is not enough space below.

Fixes: #4050