[VPAT] Fix contrast and reflow in three a11y_assessments demos - #13409

The TestFlutterView class (accessed via tester.view) does not have a general reset() method. Calling tester.view.reset will result in a compilation error. Instead, you should explicitly reset the modified properties (physicalSize and devicePixelRatio) using resetPhysicalSize() and resetDevicePixelRatio().

TestFlutterView.reset() exists (flutter_test/lib/src/window.dart) and resets physicalSize, devicePixelRatio and the other view overrides in one call. It is the pattern framework tests use with addTearDown(tester.view.reset), and this test compiles and passes on CI. Keeping it as is.

Using the force-unwrap operator ! on richText.text.style and style.color can lead to unhandled null pointer exceptions during tests if the text style or color is not explicitly set on the widget. It is safer to use null-aware operators and throw a descriptive StateError if the color cannot be resolved, which makes debugging test failures much easier.

  final RichText richText = tester.widget<RichText>(
    find.descendant(of: text, matching: find.byType(RichText)),
  );
  final Color? color = richText.text.style?.color;
  if (color == null) {
    throw StateError('The text style or color is null for the found RichText widget.');
  }
  return color;

This is a test helper. If the style or color is null the test fails right here with a stack trace naming paintedTextColor, which is the behavior we want, so the explicit StateError adds code without changing the signal. For the Material text these tests read, RichText.text.style is the merged effective style and the color is always resolved.