revert: "feat(IDP): use single callback endpoint"

This reverts commit e126ccc9aa.

# Which Problems Are Solved

#8295 introduced the possibility to handle idps on a single callback,
but broke current setups.

# How the Problems Are Solved

- Revert the change until a proper solution is found. Revert is needed
as docs were also changed.

# Additional Changes

None.

# Additional Context

- relates to #8295

cmd/start/start.go

@@ -425,6 +425,8 @@ func startAPIs(
 	assetsCache := middleware.AssetsCacheInterceptor(config.AssetStorage.Cache.MaxAge, config.AssetStorage.Cache.SharedMaxAge)
 	apis.RegisterHandlerOnPrefix(assets.HandlerPrefix, assets.NewHandler(commands, verifier, config.InternalAuthZ, id.SonyFlakeGenerator(), store, queries, middleware.CallDurationHandler, instanceInterceptor.Handler, assetsCache.Handler, limitingAccessInterceptor.Handle))
 
+	apis.RegisterHandlerOnPrefix(idp.HandlerPrefix, idp.NewHandler(commands, queries, keys.IDPConfig, config.ExternalSecure, instanceInterceptor.Handler))
+
 	userAgentInterceptor, err := middleware.NewUserAgentHandler(config.UserAgentCookie, keys.UserAgentCookieKey, id.SonyFlakeGenerator(), config.ExternalSecure, login.EndpointResources, login.EndpointExternalLoginCallbackFormPost, login.EndpointSAMLACS)
 	if err != nil {
 		return nil, err
@@ -488,8 +490,6 @@ func startAPIs(
 	apis.RegisterHandlerOnPrefix(login.HandlerPrefix, l.Handler())
 	apis.HandleFunc(login.EndpointDeviceAuth, login.RedirectDeviceAuthToPrefix)
 
-	apis.RegisterHandlerOnPrefix(idp.HandlerPrefix, idp.NewHandler(commands, queries, keys.IDPConfig, config.ExternalSecure, instanceInterceptor.Handler, login.IDPCallbackRedirect))
-
 	// After OIDC provider so that the callback endpoint can be used
 	if err := apis.RegisterService(ctx, oidc_v2.CreateServer(commands, queries, oidcServer, config.ExternalSecure)); err != nil {
 		return nil, err