Authenticates a discovery OAuth token received via deeplink after a browser-based discovery OAuth flow completes. Calls the POST /sdk/v1/b2b/oauth/discovery/authenticate endpoint. Retrieves the PKCE code verifier stored during the [B2BOAuthProviderDiscoveryClient.start] call. Returns discovered organizations and an intermediate session token; call [B2BDiscoveryIntermediateSessionsClient.exchange] to establish a full member session.