Fix: DraggableScrollableSheet may not close if snapping is enabled (#165557)

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fixes #140701

This PR fixes an issue where a DraggableScrollableSheet with `snap` set
to true and `shouldCloseOnMinExtent` set to true may not close when
dragged downward.

The issue was caused by round-off errors accumulated by `addPixelDelta`
method, which could lead to `extent.currentSize` not matching the
`extent.minSize` exactly when the bottom is reached. I added logic to
correct it when the snapping ballistic animation is complete.

<details>
<summary>Sample code</summary>

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

void main() {
  runApp(MyApp());
}

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

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: HomePage(),
    );
  }
}

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

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text("DraggableScrollableSheet Test"),
      ),
      body: Center(
        child: ElevatedButton(
          child: Text("Open"),
          onPressed: () {
            showModalBottomSheet(
              context: context,
              showDragHandle: true,
              isScrollControlled: true,
              builder: (context) => _buildBottomSheet(),
            );
          },
        ),
      ),
    );
  }

  Widget _buildBottomSheet() {
    return NotificationListener<DraggableScrollableNotification>(
      onNotification: (notification) {
        print(notification.extent);
        return false;
      },
      child: DraggableScrollableSheet(
        expand: false,
        snap: true,
        shouldCloseOnMinExtent: true,
        maxChildSize: 0.9,
        minChildSize: 0.25,
        builder: (context, scrollController) {
          return ListView.builder(
            controller: scrollController,
            itemCount: 100,
            itemBuilder: (context, index) {
              return ListTile(
                title: Text("Item $index"),
              );
            },
          );
        },
      ),
    );
  }
}
```

</details>

| Before applying fix | After |
| --- | --- |
| * Occurs with probability <video
src="https://github.com/user-attachments/assets/ffd2d097-3ed5-4775-90d5-950092d49591">
| <video
src="https://github.com/user-attachments/assets/0f20cb81-3444-40a3-a84d-ed4bff15887e">
|


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