Read the bus's release from the nats module's pinned server image, not a NATS_BASE build argument (issue 1875)

The nats module takes its server as an upstream artifact, pinned by digest
(issue 1641), and builds no image on a NATS_BASE argument any more. The test of
the bundle's bus read that argument, found none, and failed every mesh-host check
against the catalogue's main. Bootstrap's own code never read it: the bundle's
bus image comes from the bundle, so genesis was not affected. The test now
compares the bundle's image with the module's server by repository and digest.

internal/bootstrap/bus_adoption_test.go

@@ -108,28 +108,35 @@ func TestABusStepThatEndsFailedIsRefused(t *testing.T) {
 	}
 }
 
-// The bundle's bus runs the release the module's image is built on, pinned by the same digest, so the
-// adoption changes no server release and is reversible as the step says (ADR 0333 §3).
+// The bundle's bus runs the release the module's server is, pinned by the same digest, so the adoption changes no
+// server release and is reversible as the step says (ADR 0333 §3). The module takes the server as an upstream
+// artifact, its image reference pinned by digest (issue 1641); it no longer builds an image on a NATS_BASE argument,
+// which this test read until issue 1875.
 func TestTheBundlesBusRunsTheReleaseTheModulesImageIsBuiltOn(t *testing.T) {
 	var m struct {
 		Build struct {
-			On []struct{ Arg, Image string } `json:"on"`
+			Artifacts []struct{ Name, Kind, From string } `json:"artifacts"`
 		} `json:"build"`
 	}
 	if err := json.Unmarshal(theCataloguesModule(t, BusModule), &m); err != nil {
 		t.Fatal(err)
 	}
-	base := ""
-	for _, on := range m.Build.On {
-		if on.Arg == "NATS_BASE" {
-			base = on.Image
+	from := ""
+	for _, a := range m.Build.Artifacts {
+		if a.Name == "server" && a.Kind == "upstream" {
+			from = a.From
 		}
 	}
+	name, digest, pinned := strings.Cut(from, "@")
+	if !pinned || !strings.HasPrefix(digest, "sha256:") {
+		t.Fatalf("the nats module's server is no upstream image pinned by digest: %q", from)
+	}
+	repository, _, _ := strings.Cut(name, ":")
 	rewritten, err := Rewrite(theRealBundle(t), held)
 	if err != nil {
 		t.Fatal(err)
 	}
-	if got := containerNamed(rewritten.Declaration, "mesh-broker-nats").Image; got != base {
-		t.Errorf("the bundle raises the bus on %s; the nats module's image is built on %s", got, base)
+	if got, want := containerNamed(rewritten.Declaration, "mesh-broker-nats").Image, repository+"@"+digest; got != want {
+		t.Errorf("the bundle raises the bus on %s; the nats module's server is %s (%s)", got, want, from)
 	}
 }