feat(wifi_iot): add connectWithResult with native error details

[JarenFeng]

The boolean connection result does not expose the reason for a failed connection. Add connectWithResult() returning a typed WiFiConnectResult with a stable error code and available native error/status details.

Existing connect() and findAndConnect() retain their Future<bool> signatures and boolean method-channel contracts. Android's existing invalid-BSSID and unsupported-WEP exceptions are preserved for those callers.

• Return code, isSuccess, nullable nativeCode, nativeDomain and message through the Dart result type.
• Preserve Android network-suggestion status integers and iOS NSError codes, domains and localized messages.
• Report local connection failures and adapter/configuration errors using stable codes; catch native builder exceptions instead of leaving the connection future unresolved.
• Validate UTF-8 SSID length, missing passwords and timeout values before the new method invokes native code.
• Document that Android withInternet: true success means the suggestion was accepted, and that onUnavailable() cannot reliably distinguish authentication failure from user rejection or other failures.

Validation:

• 13 Flutter regression tests passed, covering the new API, native details, validation, missing plugins and existing boolean/exception contracts.
• 17 assertions against the compiled Android result adapter passed.
• Android example debug APK and iOS simulator example built with Flutter 3.47.1 / Dart 3.13.1.
• Swift implementation type-checked against the iOS simulator SDK with an iOS 13 deployment target.
• flutter analyze --no-pub, Dart format checks and project Java/Swift formatting passed.
• Both proposed PRs merge cleanly. The combined tree passed the 13 regression tests plus a WPA3 parameter interoperability test and an Android build.

Physical-device connection testing has not been performed during this preparation. This PR does not add a network handle or change the package version/release changelog.

Related: #427.