[uveshm003]

collectCorsPreflightReasons (added in #2556) lists withCredentials as a preflight reason. Credentials mode does not trigger a preflight; only a non-safelisted method, non-safelisted headers or an upload listener do (MDN, Requests with credentials: a credentialed simple GET "is not preflighted"). As a result, BrowserHttpClientAdapter(withCredentials: true) logs the "will trigger a preflight (OPTIONS) request" warning on every simple GET, and network errors on such requests get a misleading "not a CORS simple request" hint.

Fix: drop the credentials reason and the now-unused withCredentials parameter. cors.dart lives under lib/src and isn't exported, so no public API changes.

Closes #2634 (auto-closed only because the wrong issue template was used).

Tests: removed the test that asserted the wrong behaviour and the withCredentials case from the combined test. dart test -p vm test/cors_preflight_test.dart (22 tests) and dart analyze --fatal-infos pass on dio_web_adapter.

Unverified: the browser test suite (-p chrome) wasn't run locally; the adapter change is the removal of one named argument.

Implementation and tests by Claude Code (Claude Opus 5.5); reviewed and submitted by me.

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