[web_adapter] CORS warning claims withCredentials forces a preflight, so every credentialed simple request is flagged #2634

Package: web_adapter
Version: 2.2.2 (main @ f1bef46)
Operating-System: Web
Adapter: Default Dio
Dart Version: 3.12.2

collectCorsPreflightReasons treats withCredentials: true as a preflight trigger. That isn't how CORS works: preflight depends only on the method, the request headers and upload listeners, not on credentials mode (Fetch spec "main fetch" step; MDN: "a simple GET request is not preflighted" even with credentials). As a result, BrowserHttpClientAdapter(withCredentials: true) logs the "will trigger a preflight (OPTIONS) request" warning on every plain GET, and any network error on such a request gets a misleading "not a CORS simple request" hint.

Steps to Reproduce

import 'package:dio/dio.dart';
import 'package:dio_web_adapter/src/cors.dart';

void main() {
  final options = RequestOptions(path: 'https://api.example.com/me'); // simple GET
  print(collectCorsPreflightReasons(options, withCredentials: false));
  print(collectCorsPreflightReasons(options, withCredentials: true));
}

Expected Result

[]
[]

Actual Result

[]
[withCredentials is enabled, which requires a CORS preflight request]

Cause: plugins/web_adapter/lib/src/cors.dart:77-81 (used at adapter_impl.dart:85-89 for both the warning and the onError message).

Suggested fix: drop the withCredentials reason from collectCorsPreflightReasons. If a credentials hint is still useful, add it only to the onError message, worded as "the server must send Access-Control-Allow-Credentials: true and a non-wildcard origin".