Add Close() method to Dropdown components for manual dismissal

[Copilot]

Summary

• add Close() to Dropdown and DropdownField so callers can dismiss native dropdown popups before navigation
• route Close() through the handler command mapper with platform partial implementations
• keep the Android PopupMenu dismissible and clear stale menus when reopening or disconnecting the handler
• document Close() usage for Dropdown and DropdownField

Automated Testing

• dotnet build "src\UraniumUI\UraniumUI.csproj" -f net10.0
• dotnet build "src\UraniumUI\UraniumUI.csproj" -f net10.0-windows10.0.19041.0
• dotnet build "src\UraniumUI\UraniumUI.csproj" -f net10.0-android
• dotnet build "src\UraniumUI\UraniumUI.csproj" -f net10.0-ios
• dotnet build "src\UraniumUI\UraniumUI.csproj" -f net10.0-maccatalyst
• dotnet build "src\UraniumUI.Material\UraniumUI.Material.csproj" -f net10.0
• dotnet test "test\UraniumUI.Tests.sln"

Manual Testing

Not run locally. Recommended Android verification:

• Run a sample page containing a DropdownField on an Android device or emulator.
• Open the dropdown, call dropdownField.Close() before programmatic navigation, then navigate to another page.
• Expected result: the Android native popup is dismissed and is not visible on the next page.

Closes #918

[enisn]

Your PR has compilation errors @copilot

[Copilot]

Your PR has compilation errors @copilot

Fixed in c686467. The issue was that Close() is a method/command, not a property, so it needed to be added to a CommandMapper instead of the PropertyMapper. The handler now correctly uses DropdownCommandMapper with the proper MapClose(handler, dropdown, args) signature.

[enisn]

Resolved the remaining compile errors in 420ce1a by switching the platform close hook to a partial method and cleaning up Android popup dismissal on reopen/disconnect. Local targeted builds and tests pass; CI is rerunning.