Fix wide `DatePicker` input mode button padding for Material 3

fixes Wide DatePickerDialog "Switch to input" button has no margin from dialog side

Description

Currently, there is no material design token for the input mode button padding. This PR fixes the padding by taking date picker side padding values into account.

specs

Code sample

expand to view the code sample

```dart import 'package:flutter/material.dart'; void main() { runApp(const MyApp()); } class MyApp extends StatelessWidget { const MyApp({super.key}); @override Widget build(BuildContext context) { return MaterialApp( home: Scaffold( body: Center( child: Builder(builder: (BuildContext context) { return ElevatedButton( onPressed: () { showDatePicker( context: context, initialDate: DateTime.now(), firstDate: DateTime.utc(2010), lastDate: DateTime.utc(2030), ); }, child: const Text('Show Date Picker'), ); }), ), ), ); } } ```

Before

before

After

after

Pre-launch Checklist

[x] I read the Contributor Guide and followed the process outlined there for submitting PRs.
[x] I read the Tree Hygiene wiki page, which explains my responsibilities.
[x] I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
[x] I signed the CLA.
[x] I listed at least one issue that this PR fixes in the description above.
[ ] I updated/added relevant documentation (doc comments with ///).
[x] I added new tests to check the change I am making, or this PR is test-exempt.
[ ] I followed the breaking change policy and added Data Driven Fixes where supported.
[x] All existing and new tests are passing.

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flutter / flutter