feat(IDP): use single callback endpoint (#8295)

# Which Problems Are Solved

Both the login UI and the IdP intent flow have their own IdP callback
endpoints.

This makes configuration hard to impossible (e.g. Github only allows one
endpoint) for customers.

# How the Problems Are Solved

- The login UI prefixes the `state` parameter when creating an auth /
SAML request.
- All requests now use the `/idp/callback` or the corresponding
variation (e.g. SAML)
- On callback, the state, resp. its prefix is checked. In case of the
login UI prefix, the request will be forwarded to the existing login UI
handler without the prefix state.
Existing setups will therefore not be affected and also requests started
before this release can be handled without any impact.
- Console only lists the "new" endpoint(s). Any
`/login/externalidp/callback` is removed.

# Additional Changes

- Cleaned up some images  from the IdP documentation.
- fix the error handling in `handleExternalNotFoundOptionCheck`

# Additional Context

- closes #8236

cmd/start/start.go

@@ -425,8 +425,6 @@ 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
@@ -490,6 +488,8 @@ 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