Fix: Correct semantic binding issue

[WillCallahan]

This PR resolves a critical accessibility issue where the country selector button within PhoneFormField was inaccessible to screen reader users.

How to Reproduce the Error

The issue can be reproduced by enabling semantics in your app, for example, by adding the following lines to your main.dart:

void main() {
  WidgetsFlutterBinding.ensureInitialized();
  SemanticsBinding.instance.ensureSemantics();
  runApp(const MyApp());
}

With semantics enabled, the country selector button was not focusable or interactive for screen readers.

The Fix

The problematic PhoneFieldSemantics widget has been removed. The CountryButton now provides its own correct semantic information (descriptive label, button role), allowing screen readers to interact with it properly.

Impact

Screen reader users can now correctly focus, understand, and activate the country selector.

Testing

To test this change:

1. Enable a screen reader (e.g., VoiceOver/TalkBack).
2. Navigate to a PhoneFormField.
3. Verify the screen reader announces the country selector button descriptively (e.g., "United States +1, button").
4. Verify the button can be activated via the screen reader's activation gesture.

Related Issues

• Closes Semantics not working #340

Checklist

• Fix: Enable typing in search field when semantics are enabled flutter_country_selector#37 has been merged
• I have bumped the version in pubspec.yaml
• I have added an entry in changelog.md for the new pubspec version
• I have added "Closes Semantics not working #340" to this template to automatically close related issues.
• (not required if no sensible change has been made eg: Localization) I have added tests that prove my fix is effective or that my feature works.

Supporting Documentation

• Semantics Widget: The primary widget for providing accessibility information.
• ExcludeSemantics Widget: The widget that was causing the issue by hiding the TextField from assistive technologies.
• An introduction to accessibility in Flutter: General guidance and best practices for making Flutter applications accessible.

[MahMoos]

@cedvdb can we have this merged and released please?

[WillCallahan]

I have made the required updates and tested this