fix(windows): position child host windows in parent-client coordinates

[palmoni5]

Problem

Porting upstream PR pichillilorenzo#2768 (c568a24e) turned the three internal WebView host HWNDs into WS_CHILD windows. InAppWebView::setPosition was not adjusted and kept passing screen coordinates to SetWindowPos:

GetWindowRect(flutterWindowHWnd, &flutterWindowRect);
::SetWindowPos(webViewHWnd, nullptr,
  flutterWindowRect.left + scaled_x - borderWidth,
  flutterWindowRect.top  + scaled_y - titleBarHeight, ...);

That was correct for the previous owned top-level host. For a child window SetWindowPos takes coordinates relative to the parent's client area, so the host window ends up offset by the Flutter window's screen origin.

Everything the host window owns moves with it: <select> dropdowns, the print preview printer/color pickers, and context menus all open away from the WebView. Because the error tracks the window position, it changes as the window is moved or resized and reads as intermittent. Keyboard navigation is unaffected (no coordinates involved), which hides the cause.

Measured on a real app window (client origin (42,133)), the host HWND sat at (79,275) — offset by exactly the window origin plus the frame fudge.

Fix

Pass the widget offset as-is. This also removes the hand-rolled frame arithmetic (GetWindowRect + SM_CYCAPTION/SM_CYFRAME/SM_CXPADDEDBORDER), which mis-measured borderless windows and was never DPI-correct — it used GetSystemMetrics rather than GetSystemMetricsForDpi.

::SetWindowPos(webViewHWnd, nullptr, scaled_x, scaled_y, 0, 0,
  SWP_NOZORDER | SWP_NOSIZE | SWP_NOACTIVATE);

Testing

• native_host_window_position_test.dart — new, source-scanning in the same style as native_host_window_style_test.dart: asserts setPosition positions by scaled_x/scaled_y and does not reintroduce screen-space APIs.
• Verified at runtime in a Flutter Windows host: <select> dropdowns and the WebView2 print preview pickers open in the right place again.

Note for hosts on a mirrored window: a parent with WS_EX_LAYOUTRTL measures child coordinates from the right edge, so that style has to be off for child-relative positioning to land correctly.