Bridge self-contained legacy OAuth code exchange overrides

Opt-in ASP.NET Core 2.x to 3.0+ signature port: retain a directly derived OAuthHandler's two-string ExchangeCodeAsync implementation as a protected virtual compatibility method and add the context override forwarding Code and RedirectUri. Preserve the original implementation, parameter contracts, documentation and virtual string dispatch. Leave base calls, additional exchange overloads, method groups, partial classes and conditional snapshots unchanged. Does not add PKCE or port JSON APIs; authentication properties stay on the framework's context and are never fabricated.