feat(auth): pass id_token_hint on Keycloak logout

Carries the raw ID token in the session cookie so logout can hand it back
to Keycloak as id_token_hint, letting it skip its own logout-confirmation
prompt -- otherwise a user could cancel out of it and land back in the app
with a Keycloak SSO session but no geniusrun profile (e.g. right after
deleting their account).

backend/internal/auth/oidc_test.go

@@ -1,10 +1,83 @@
 package auth
 
 import (
+	"context"
 	"encoding/json"
+	"fmt"
+	"net/http"
+	"net/http/httptest"
+	"net/url"
 	"testing"
 )
 
+// newTestOIDCVerifier spins up a fake OIDC discovery endpoint (just enough
+// for oidc.NewProvider's discovery GET to succeed) and returns a real
+// oidcVerifier backed by it, for tests that exercise EndSessionURL without
+// a live Keycloak.
+func newTestOIDCVerifier(t *testing.T) Verifier {
+	t.Helper()
+	var issuerURL string
+	mux := http.NewServeMux()
+	mux.HandleFunc("/.well-known/openid-configuration", func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		fmt.Fprintf(w, `{
+			"issuer": %[1]q,
+			"authorization_endpoint": "%[1]s/auth",
+			"token_endpoint": "%[1]s/token",
+			"end_session_endpoint": "%[1]s/logout",
+			"jwks_uri": "%[1]s/certs"
+		}`, issuerURL)
+	})
+	mux.HandleFunc("/certs", func(w http.ResponseWriter, r *http.Request) {
+		w.Header().Set("Content-Type", "application/json")
+		fmt.Fprint(w, `{"keys":[]}`)
+	})
+	srv := httptest.NewServer(mux)
+	t.Cleanup(srv.Close)
+	issuerURL = srv.URL
+
+	verifier, err := NewOIDCVerifier(context.Background(), OIDCConfig{
+		IssuerURL: issuerURL, ClientID: "geniusrun", ClientSecret: "secret", RedirectURL: issuerURL + "/callback",
+	})
+	if err != nil {
+		t.Fatalf("NewOIDCVerifier: %v", err)
+	}
+	return verifier
+}
+
+func TestEndSessionURL_IncludesIDTokenHintWhenProvided(t *testing.T) {
+	verifier := newTestOIDCVerifier(t)
+
+	got := verifier.EndSessionURL("https://app.example.com/", "raw-id-token-jwt")
+	u, err := url.Parse(got)
+	if err != nil {
+		t.Fatalf("parse EndSessionURL result %q: %v", got, err)
+	}
+	q := u.Query()
+	if q.Get("id_token_hint") != "raw-id-token-jwt" {
+		t.Errorf("id_token_hint = %q, want %q", q.Get("id_token_hint"), "raw-id-token-jwt")
+	}
+	if q.Get("client_id") != "geniusrun" {
+		t.Errorf("client_id = %q, want geniusrun", q.Get("client_id"))
+	}
+	if q.Get("post_logout_redirect_uri") != "https://app.example.com/" {
+		t.Errorf("post_logout_redirect_uri = %q, want https://app.example.com/", q.Get("post_logout_redirect_uri"))
+	}
+}
+
+func TestEndSessionURL_OmitsIDTokenHintWhenEmpty(t *testing.T) {
+	verifier := newTestOIDCVerifier(t)
+
+	got := verifier.EndSessionURL("https://app.example.com/", "")
+	u, err := url.Parse(got)
+	if err != nil {
+		t.Fatalf("parse EndSessionURL result %q: %v", got, err)
+	}
+	if u.Query().Has("id_token_hint") {
+		t.Errorf("expected no id_token_hint param when idTokenHint is empty, got %q", got)
+	}
+}
+
 func TestIDTokenClaims_HasRole(t *testing.T) {
 	cases := []struct {
 		name string