Multiple improvements for April 2025

* Improve refresh token handling in the background.

Resolves issue when user opens the website, allows the access token to expire, but continues browsing.
The background requests are failing with CORS errors to OIDC provider.

* fixup! Improve refresh token handling in the background.

* Abstract the token blacklisting.

session_test.go

@@ -1,7 +1,9 @@
 package traefikoidc
 
 import (
-	"math/rand"
+	"crypto/rand"
+	"fmt"
+	"math/big"
 	"net/http/httptest"
 	"strings"
 	"testing"
@@ -12,7 +14,12 @@ func generateRandomString(length int) string {
 	const charset = "abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789"
 	b := make([]byte, length)
 	for i := range b {
-		b[i] = charset[rand.Intn(len(charset))]
+		num, err := rand.Int(rand.Reader, big.NewInt(int64(len(charset))))
+		if err != nil {
+			// Handle error appropriately in a real application, maybe panic in test helper
+			panic(fmt.Sprintf("crypto/rand failed: %v", err))
+		}
+		b[i] = charset[num.Int64()]
 	}
 	return string(b)
 }