mesh-grants: give the enrolment fixture its tunnel key, which an enrolment now requires

mesh-sdk 22c03af (feat/l3-enrol) requires overlay-key on every enrolment; the fixture of the enrolment-user subtest carries one.

(issue 774, review round 1b)

Claude-Session: https://claude.ai/code/session_01NcbgfJ9Gum47zkFKWRaovL

modules/mesh-grants/grants_test.go

@@ -3,6 +3,7 @@ package manifest
 import (
 	"bytes"
 	"context"
+	"encoding/base64"
 	"encoding/json"
 	"errors"
 	"fmt"
@@ -2425,7 +2426,8 @@ func TestGrants(t *testing.T) {
 		issued := time.Date(2026, 11, 2, 1, 0, 0, 0, time.UTC)
 		for node, stage := range map[string]string{"lab": delivery.EnrolmentLive, "pi": delivery.EnrolmentAsked} {
 			raw, err := delivery.EncodeEnrolment(delivery.Enrolment{Node: node, Address: "10.0.0.7", Why: "joins",
-				Caller: "a.mesh-enrolment", Stage: stage, Issued: issued, Expires: issued.Add(time.Hour)})
+				Caller: "a.mesh-enrolment", Stage: stage, Issued: issued, Expires: issued.Add(time.Hour),
+				OverlayKey: base64.StdEncoding.EncodeToString(make([]byte, 32))})
 			if err != nil {
 				t.Fatal(err)
 			}