fix(native_dio_adapter): gracefully fall back when Cronet unavailable

[DevMohammadSalameh]

Summary

Fixes #2444

When Google Play Services is unavailable (e.g., on Android emulators without Google APIs or devices without Play Services), CronetEngine initialization throws a JniException:

"All available Cronet providers are disabled. A provider should be enabled before it can be used."

This change:

• Wraps CronetAdapter creation in a try-catch block
• Falls back to the standard HttpClientAdapter (dart:io) when Cronet is unavailable
• Adds an optional onCronetUnavailable callback for developers who want to be notified when the fallback occurs

Changes

• native_adapter.dart: Added graceful fallback with try-catch
• CHANGELOG.md: Documented the new behavior

Usage

// Basic usage - fallback happens silently
final dio = Dio();
dio.httpClientAdapter = NativeAdapter();

// With callback to log or handle the fallback
dio.httpClientAdapter = NativeAdapter(
  onCronetUnavailable: (error) {
    print('Cronet unavailable, using dart:io fallback: $error');
  },
);

Test Plan

• melos run format - passes
• melos run analyze - passes
• flutter test in native_dio_adapter - all 10 tests pass
• Manual test on Android emulator without Google Play Services (requires device testing)