fix(passkeys_darwin): forward userVerification preference to AuthenticationServices

[spydon]

Summary

• `register` and `authenticate` on iOS and macOS accepted a user verification preference (`authSelectionType.userVerification` and `AuthenticateRequestType.userVerification`) but never forwarded it, so Apple's default of `preferred` always applied.
• Adds a `userVerificationPreference` parameter to the darwin Pigeon API and forwards it from the Dart layer.
• Maps `required`, `preferred` and `discouraged` onto `userVerificationPreference` for the platform and security key registration and assertion requests. Unknown or missing values leave the system default untouched, matching the previous behaviour.
• Adds unit tests asserting the value is forwarded (and is null when unset) for both operations.

Closes #304

Test plan

• `melos format --set-exit-if-changed`
• `flutter analyze --fatal-infos` and `flutter test` in `passkeys_darwin`
• Typechecked the AuthenticationServices usage against the macOS SDK
• iOS SwiftPM, CocoaPods and previous Xcode builds via CI (no Xcode on the local machine)