Only review question: service interface or extension methods?

I don't have a strong opinion on service vs extension method.

Pro/con of service:
Con: Service suggests an extensibility point where none is intended
Pro: Service might be easier to mock

Con: Service suggests an extensibility point where none is intended

Right... this was the main thing I was worried about. Also con, if the implementation needs more stuff from DI (e.g. logger) then it makes more sense as a service.

Anyway, your call.

We do have a lot of dependencies, so I'm leaning away from extension methods actually:

    IOptions<UserTokenManagementOptions> tokenManagementOptions, // TODO - Consider option lifetime
    IAuthenticationSchemeProvider schemeProvider,
    ILogger<StoreTokensInAuthenticationProperties> logger,
    IOptionsMonitor<CookieAuthenticationOptions> cookieOptionsMonitor