Offer the catalogue forge's provisions in the toolchain test's stand-in (issue 1966)

The group's mesh-build-go requires go-package-registry, which the forge
now provides; the stand-in listed the forge's offers by hand and missed it,
so the test failed against the group's catalogue. It now takes them from
the catalogue's gitea manifest, so a registry the forge gains needs no edit.

Claude-Session: https://claude.ai/code/session_01JG73KG7AfSdh6FP8VrBqCn

internal/compose/resolution/toolchain_manifests_test.go

@@ -18,8 +18,14 @@ func builderNode() mcat.Node {
 
 func toolchainShelf(t *testing.T) map[string]mcat.Manifest {
 	t.Helper()
+	// The stand-in offers by name what the catalogue's forge offers, so a registry the forge gains is offered
+	// here too and never has to be added by hand, plus the two it stands in for that the forge does not offer.
+	offers := []mcat.Offer{{Name: "artifact-store"}, {Name: "node-tools"}}
+	for _, o := range catalogueManifest(t, "gitea").Provides {
+		offers = append(offers, mcat.Offer{Name: o.Name})
+	}
 	m := map[string]mcat.Manifest{
-		"forge": {Module: "forge", Version: "1", Provides: []mcat.Offer{{Name: "forge-account"}, {Name: "npm-package-registry"}, {Name: "artifact-store"}, {Name: "node-tools"}}},
+		"forge": {Module: "forge", Version: "1", Provides: offers},
 	}
 	for _, name := range []string{"go", "nodejs", "mesh-build-go", "mesh-build-node", "dev-tools"} {
 		m[name] = catalogueManifest(t, name)