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)

[AlexV525]

As #2444 (comment) said, I think this should be set as opt-in.

[AlexV525]

Suggested change

	} catch (e) {
	// Cronet requires Google Play Services. Fall back to dart:io
	// HttpClient when unavailable (e.g., on emulators without Google APIs
	// or devices without Google Play Services).
	onCronetUnavailable?.call(e);
	} catch (e, s) {
	// Cronet requires Google Play Services. Fall back to dart:io
	// HttpClient when unavailable (e.g., on emulators without Google APIs
	// or devices without Google Play Services).
	onCronetUnavailable?.call(e, s);

[AlexV525]

Superseded by #2573, which takes a different approach based on the design in docs/plans/cronet-provider-fallback.md:

• Opt-in, not default: existing apps are unaffected until they explicitly pass createFallbackAdapter. This avoids silently changing the networking stack (TLS/proxy/cookies/protocols/pooling) for apps that never asked for it.
• Factory returns any HttpClientAdapter (not a fixed IOHttpClientAdapter), so callers can pick an adapter matching their TLS, proxy, cookie, transport, and observability requirements.
• Classifier matches the exact Chromium provider-disabled RuntimeException on a typed JniException (adds jni as a direct dep) instead of a broad try/catch — post-init connection/TLS/timeout/redirect/cancellation errors remain Cronet errors.
• Detection happens lazily on the first fetch, since CronetEngine.build() is deferred to first request; a constructor-level try/catch would miss it.
• Selection is sticky and close before any request never initializes Cronet.

Thanks for the initial PR and for surfacing the fix — it prompted the design discussion. Closing this in favor of #2573.