Implement passkey registration and authentication using Android Credential Manager API

Steps

1. Add the androidx.credentials:credentials dependency (and credentials-play-services-auth for Google Play Services compatibility) to your Android project
2. Fetch registration options from your server's /webauthn/register/begin endpoint, then call credentialManager.createCredential() with a CreatePublicKeyCredentialRequest built from the options JSON
3. Parse the CreatePublicKeyCredentialResponse JSON and POST the attestation response to your server's /webauthn/register/finish endpoint for verification
4. For authentication, fetch challenge options from /webauthn/login/begin, then call credentialManager.getCredential() with a GetPublicKeyCredentialOption; handle GetCredentialException.TYPE_NO_CREDENTIAL for the case where no passkey exists
5. Parse the GetCredentialResponse, extract the PublicKeyCredential, and POST the assertion response to your server for verification
6. Handle the credential provider selection UX: Credential Manager aggregates from Google Password Manager and any installed third-party providers (1Password, Samsung Pass etc.) — the system bottom sheet handles selection automatically