feat: warn about CORS preflight requests on Web

[CaiJingLong]

Summary

• Detect when a request is not a CORS "simple request" and will trigger a preflight (OPTIONS) request on the Web platform, then emit a warningLog explaining why (non-safelisted method, custom headers, non-simple Content-Type, upload progress listener, or withCredentials).
• Enrich the XMLHttpRequest.onError error message with CORS guidance when the request was preflighted, so users see a actionable hint instead of a generic network error.
• Extract the pure detection logic into lib/src/cors.dart (corsPreflightReason) so it can be unit-tested on the VM without a browser.

Closes #2201.

Test plan

• dart analyze — no issues
• dart format — applied
• dart test test/cors_preflight_test.dart -p vm — 15 tests pass
• Browser CI (CORS preflight tests in dio_test) — existing corsTests still cover runtime behavior

Generated with Devin

[github-actions]

Code Coverage Report: Only Changed Files listed

Package	Base Coverage	New Coverage	Difference
plugins/web_adapter/lib/src/adapter_impl.dart	🔴 19.27%	🔴 22.41%	🟢 3.14%
plugins/web_adapter/lib/src/cors.dart	🔴 0%	🟢 100%	🟢 100%
Overall Coverage	🟢 85.55%	🟢 85.61%	🟢 0.06%

Minimum allowed coverage is 0%, this run produced 85.61%