WithTags and other OpenApiRouteHandlerBuilderExtensions don't work with route groups today because they target a custom IEndpointConventionBuilder type (RouteHandlerBuilder) rather than any IEndpointConventionBuilder. While we do plan to support custom convention builder types on route groups (#41427), it shouldn't be necessary for many of the methods on OpenApiRouteHandlerBuilderExtensions.

var group = app.MapGroup("/todos");
group.WithTags("tag"); // This doesn't compile 
group.WithDescription("description"); // ditto
group.WithSummary("summary"); // ditto

Describe the solution you'd like

We should retarget WithTags and similar methods to IEndpointConventionBuilder. If we cannot do that without breaking, maybe it should target GroupRouteBuilder (or a future IRouteGroup interface).