Fix merging routes when nesting

[xitep]

I have two (or more) (openapi-)routers (from utoipa-axum) which both/all get nested into a top-level router using the same path prefix. (the reason is that i apply different (service) layers to the routers before assembling them.) however, while the axum router "merge" upon nesting works as expected, the openapi-router simply discards the old "paths" from openapi. this should correct the situation.

a possible workaround is:

let routes_1: OpenApiRouter = ...;
let routes_2: OpenApiRouter = ...;

let routes_1 = OpenApiRouter::default().nest("/foo", routes_1);
let routes_2 = OpenApiRouter::default().nest("/foo", routes_2);
let all_routes = routes1.merge(routes2);

or with vanilla utoipa:

        let mut all_routes = OpenApiBuilder::default()
            .build()
            .nest("/foo", routes1);
        routes.merge(
            OpenApiBuilder::default()
                .build()
                .nest("/foo", routes_2),
        );