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

internal/api/idp/idp.go

@@ -9,13 +9,11 @@ import (
 	"io"
 	"net/http"
 
-	"github.com/crewjam/saml"
 	"github.com/gorilla/mux"
 	"github.com/zitadel/logging"
 
 	"github.com/zitadel/zitadel/internal/api/authz"
 	http_utils "github.com/zitadel/zitadel/internal/api/http"
-	"github.com/zitadel/zitadel/internal/api/ui/login"
 	"github.com/zitadel/zitadel/internal/command"
 	"github.com/zitadel/zitadel/internal/crypto"
 	"github.com/zitadel/zitadel/internal/form"
@@ -53,13 +51,13 @@ const (
 )
 
 type Handler struct {
-	commands            *command.Commands
-	queries             *query.Queries
-	parser              *form.Parser
-	encryptionAlgorithm crypto.EncryptionAlgorithm
-	callbackURL         func(ctx context.Context) string
-	samlRootURL         func(ctx context.Context, idpID string) string
-	loginSAMLRootURL    func(ctx context.Context) string
+	commands                *command.Commands
+	queries                 *query.Queries
+	parser                  *form.Parser
+	encryptionAlgorithm     crypto.EncryptionAlgorithm
+	callbackURL             func(ctx context.Context) string
+	samlRootURL             func(ctx context.Context, idpID string) string
+	loginUICallbackRedirect func(w http.ResponseWriter, r *http.Request, state string) bool
 }
 
 type externalIDPCallbackData struct {
@@ -91,27 +89,22 @@ func SAMLRootURL(externalSecure bool) func(ctx context.Context, idpID string) st
 	}
 }
 
-func LoginSAMLRootURL(externalSecure bool) func(ctx context.Context) string {
-	return func(ctx context.Context) string {
-		return http_utils.BuildOrigin(authz.GetInstance(ctx).RequestedHost(), externalSecure) + login.HandlerPrefix + login.EndpointSAMLACS
-	}
-}
-
 func NewHandler(
 	commands *command.Commands,
 	queries *query.Queries,
 	encryptionAlgorithm crypto.EncryptionAlgorithm,
 	externalSecure bool,
 	instanceInterceptor func(next http.Handler) http.Handler,
+	loginUICallbackRedirect func(w http.ResponseWriter, r *http.Request, state string) bool,
 ) http.Handler {
 	h := &Handler{
-		commands:            commands,
-		queries:             queries,
-		parser:              form.NewParser(),
-		encryptionAlgorithm: encryptionAlgorithm,
-		callbackURL:         CallbackURL(externalSecure),
-		samlRootURL:         SAMLRootURL(externalSecure),
-		loginSAMLRootURL:    LoginSAMLRootURL(externalSecure),
+		commands:                commands,
+		queries:                 queries,
+		parser:                  form.NewParser(),
+		encryptionAlgorithm:     encryptionAlgorithm,
+		callbackURL:             CallbackURL(externalSecure),
+		loginUICallbackRedirect: loginUICallbackRedirect,
+		samlRootURL:             SAMLRootURL(externalSecure),
 	}
 
 	router := mux.NewRouter()
@@ -189,22 +182,6 @@ func (h *Handler) handleMetadata(w http.ResponseWriter, r *http.Request) {
 	}
 	metadata := sp.ServiceProvider.Metadata()
 
-	for i, spDesc := range metadata.SPSSODescriptors {
-		spDesc.AssertionConsumerServices = append(
-			spDesc.AssertionConsumerServices,
-			saml.IndexedEndpoint{
-				Binding:  saml.HTTPPostBinding,
-				Location: h.loginSAMLRootURL(ctx),
-				Index:    len(spDesc.AssertionConsumerServices) + 1,
-			}, saml.IndexedEndpoint{
-				Binding:  saml.HTTPArtifactBinding,
-				Location: h.loginSAMLRootURL(ctx),
-				Index:    len(spDesc.AssertionConsumerServices) + 2,
-			},
-		)
-		metadata.SPSSODescriptors[i] = spDesc
-	}
-
 	buf, _ := xml.MarshalIndent(metadata, "", "  ")
 	w.Header().Set("Content-Type", "application/samlmetadata+xml")
 	_, err = w.Write(buf)
@@ -218,6 +195,9 @@ func (h *Handler) handleACS(w http.ResponseWriter, r *http.Request) {
 	ctx := r.Context()
 	data := parseSAMLRequest(r)
 
+	if h.loginUICallbackRedirect(w, r, data.RelayState) {
+		return
+	}
 	provider, err := h.getProvider(ctx, data.IDPID)
 	if err != nil {
 		http.Error(w, err.Error(), http.StatusBadRequest)
@@ -272,6 +252,9 @@ func (h *Handler) handleCallback(w http.ResponseWriter, r *http.Request) {
 		http.Error(w, err.Error(), http.StatusBadRequest)
 		return
 	}
+	if h.loginUICallbackRedirect(w, r, data.State) {
+		return
+	}
 	intent, err := h.commands.GetActiveIntent(ctx, data.State)
 	if err != nil {
 		if zerrors.IsNotFound(err) {