mesh-build-go reads a compose ask's gzip-packed request (issue 1966)

The checker sends a large node's request packed, as it passes the bus's message limit; compose reads either form through
ComposeJob.RequestBytes. Only mesh-build-go serves compose. Delivered as a group with the mesh-delivery change of the same
branch name: an older mesh-build-go refuses a packed ask naming that it holds no request object.

Claude-Session: https://claude.ai/code/session_01NcbgfFfQ4Z9UE5jK32PmE5

modules/mesh-build-go/cmd/mesh-build-go/compose.go

@@ -62,7 +62,11 @@ func (m *Module) compose(ctx context.Context, fetch buildstep.StepFunc, a sdkste
 	if err != nil {
 		return nil, err
 	}
-	req, err := composer.Decode(job.Request)
+	body, err := job.RequestBytes()
+	if err != nil {
+		return nil, buildstep.Refuse("the compose ask %s: %v", a.ID, err)
+	}
+	req, err := composer.Decode(body)
 	if err != nil {
 		return nil, buildstep.Refuse("the compose ask %s: %v", a.ID, err)
 	}

modules/mesh-build-go/cmd/mesh-build-go/compose_test.go

@@ -81,6 +81,10 @@ func TestAKeptComposerIsRunWithTheManifestsFilled(t *testing.T) {
 	}
 	fetched := 0
 	fetch := func(context.Context, map[string]any) (any, error) { fetched++; return nil, nil }
+	// Issue 1966: the request arrives gzip-packed, as the checker sends one over the bus's message limit.
+	if err := a.Compose.PackRequest(a.Compose.Request); err != nil {
+		t.Fatal(err)
+	}
 	out, err := m.compose(context.Background(), fetch, a)
 	if err != nil {
 		t.Fatal(err)