test: fix SAML integration tests (#11241)

# Which Problems Are Solved

Some SAML integration tests used a hardcoded certificate, which expired
on the 25th December 2025.

# How the Problems Are Solved

Added helper functions to the integration package to create and store a
private key and certificate for SAML, which can be used in SAML SP/IDP
integration tests.

# Additional Changes

none

# Additional Context

- noted on #11196
- requires backport to at least v4.x

internal/integration/instance.go

@@ -4,6 +4,8 @@ package integration
 import (
 	"bytes"
 	"context"
+	"crypto/rsa"
+	"crypto/x509"
 	_ "embed"
 	"errors"
 	"fmt"
@@ -94,6 +96,10 @@ type Instance struct {
 
 	Client   *Client
 	WebAuthN *webauthn.Client
+	SAML     struct {
+		PrivateKey  *rsa.PrivateKey
+		Certificate *x509.Certificate
+	}
 }
 
 // NewInstance returns a new instance that can be used for integration tests.