[client] Populate the pending extend flow in the test server helper

SwitchProfile cancels and clears the pending session extend flow
unconditionally, the same way it clears the SSH JWT cache. New always
populates the field, but the hand-assembled test server did not, so
TestSwitchProfile_ClearsJWTCache panicked on a nil PendingFlow.

client/server/server_connect_test.go

@@ -10,6 +10,7 @@ import (
 	"github.com/stretchr/testify/require"
 
 	"github.com/netbirdio/netbird/client/internal"
+	"github.com/netbirdio/netbird/client/internal/auth"
 	"github.com/netbirdio/netbird/client/internal/peer"
 	"github.com/netbirdio/netbird/client/proto"
 )
@@ -21,7 +22,8 @@ func newTestServer() *Server {
 		// New always populates the SSH JWT cache and the logout and
 		// profile-switch paths call into it unconditionally, so a Server
 		// assembled field by field has to populate it too.
-		jwtCache: newJWTCache(),
+		jwtCache:              newJWTCache(),
+		extendAuthSessionFlow: auth.NewPendingFlow(),
 	}
 }