Fix Scrollbar.thickness property is ignored when the Scrollbar is hovered (#144012)

fixes [`Scrollbar.thickness` property is ignored when the `Scrollbar` is hovered](https://github.com/flutter/flutter/issues/143881)

### Code sample

<details>
<summary>expand to view the code sample</summary> 

```dart
import 'package:flutter/material.dart';

void main() => runApp(const MyApp());

class MyApp extends StatelessWidget {
  const MyApp({super.key});

  @override
  Widget build(BuildContext context) {
    final ScrollController scrollController = ScrollController();

    return MaterialApp(
      debugShowCheckedModeBanner: false,
      home: Material(
        child: ScrollConfiguration(
          behavior: const NoScrollbarBehavior(),
          child: ScrollbarTheme(
            data: ScrollbarThemeData(
              thickness: MaterialStateProperty.all(25.0),
              showTrackOnHover: true,
            ),
            child: Scrollbar(
              thickness: 50.0,
              thumbVisibility: true,
              radius: const Radius.circular(3.0),
              controller: scrollController,
              child: SingleChildScrollView(
                controller: scrollController,
                child: const SizedBox(width: 4000.0, height: 4000.0),
              ),
            ),
          ),
        ),
      ),
    );
  }
}

class NoScrollbarBehavior extends ScrollBehavior {
  const NoScrollbarBehavior();

  @override
  Widget buildScrollbar(
          BuildContext context, Widget child, ScrollableDetails details) =>
      child;
}
```

</details>

### Preview

| Before | After |
| --------------- | --------------- |
| <img src="https://github.com/flutter/flutter/assets/48603081/3537d60d-a5b2-488d-aa99-4c36c3721657"  /> | <img src="https://github.com/flutter/flutter/assets/48603081/cfd02095-a327-4b16-8ece-0d1c9e6813ce" /> |

packages/flutter/lib/src/material/scrollbar.dart

@@ -326,7 +326,8 @@ class _MaterialScrollbarState extends RawScrollbarState<_MaterialScrollbar> {
   MaterialStateProperty<double> get _thickness {
     return MaterialStateProperty.resolveWith((Set<MaterialState> states) {
       if (states.contains(MaterialState.hovered) && _trackVisibility.resolve(states)) {
-        return _scrollbarTheme.thickness?.resolve(states)
+        return widget.thickness
+          ?? _scrollbarTheme.thickness?.resolve(states)
           ?? _kScrollbarThicknessWithTrack;
       }
       // The default scrollbar thickness is smaller on mobile.

packages/flutter/test/material/scrollbar_theme_test.dart

@@ -362,6 +362,7 @@ void main() {
 
   testWidgets('Scrollbar widget properties take priority over theme', (WidgetTester tester) async {
     const double thickness = 4.0;
+    const double edgeMargin = 2.0;
     const bool showTrackOnHover = true;
     const Radius radius = Radius.circular(3.0);
     final ScrollController scrollController = ScrollController();
@@ -394,14 +395,14 @@ void main() {
       find.byType(Scrollbar),
       paints..rrect(
         rrect: RRect.fromRectAndRadius(
-          const Rect.fromLTRB(794.0, 0.0, 798.0, 90.0),
+          const Rect.fromLTRB(800 - thickness - edgeMargin, 0.0, 798.0, 90.0),
           const Radius.circular(3.0),
         ),
         color: _kDefaultIdleThumbColor,
       ),
     );
 
-    // Drag scrollbar behavior
+    // Drag scrollbar behavior.
     const double scrollAmount = 10.0;
     final TestGesture dragScrollbarGesture = await tester.startGesture(const Offset(797.0, 45.0));
     await tester.pumpAndSettle();
@@ -410,7 +411,7 @@ void main() {
       find.byType(Scrollbar),
       paints..rrect(
         rrect: RRect.fromRectAndRadius(
-          const Rect.fromLTRB(794.0, 0.0, 798.0, 90.0),
+          const Rect.fromLTRB(800 - thickness - edgeMargin, 0.0, 798.0, 90.0),
           const Radius.circular(3.0),
         ),
         // Drag color
@@ -423,7 +424,7 @@ void main() {
     await dragScrollbarGesture.up();
     await tester.pumpAndSettle();
 
-    // Hover scrollbar behavior
+    // Hover scrollbar behavior.
     final TestGesture gesture = await tester.createGesture(kind: ui.PointerDeviceKind.mouse);
     await gesture.addPointer();
     await gesture.moveTo(const Offset(794.0, 5.0));
@@ -433,18 +434,18 @@ void main() {
       find.byType(Scrollbar),
       paints
         ..rect(
-          rect: const Rect.fromLTRB(784.0, 0.0, 800.0, 600.0),
+          rect: const Rect.fromLTRB(792.0, 0.0, 800.0, 600.0),
           color: const Color(0x08000000),
         )
         ..line(
-          p1: const Offset(784.0, 0.0),
+          p1: const Offset(792.0, 0.0),
-          p2: const Offset(784.0, 600.0),
+          p2: const Offset(792.0, 600.0),
           strokeWidth: 1.0,
           color: const Color(0x1a000000),
         )
         ..rrect(
           rrect: RRect.fromRectAndRadius(
-            const Rect.fromLTRB(786.0, 10.0, 798.0, 100.0),
+            const Rect.fromLTRB(800 - thickness - edgeMargin, 10.0, 798.0, 100.0),
             const Radius.circular(3.0),
           ),
           // Hover color