Asking a tool goes through the seam, and loses two problems

Step 3.4. On the new bus there is no reply queue to declare and no
correlation to check: each account is granted one inbox prefix and no
other, so an answer cannot reach the wrong asker. That settles a cost
build.go records having paid — on a shared reply exchange every asker saw
every result, which is why the correlation was checked rather than assumed.

And a tool nobody serves says so at once rather than after the whole wait.
The difference between "that module is down" and "that tool is slow" is the
first thing a person asking wants, and both tests are against a real server
because both are claims about what the server does, not about this code.

RequestBuild stays as it is, and is a different shape on the new bus rather
than the same one: a build takes minutes, so it is work submitted to a
queue with the outcome returning to a reply subject the request carries —
the pattern design 25 §2 already sets for anything crossing a stream. It
touches the builder too, so it goes with that conversion.

internal/link/bus_test.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"encoding/json"
 	"os"
+	"strings"
 	"testing"
 	"time"
 
@@ -93,3 +94,61 @@ func TestAnEventsSubjectIsDerivedFromItsSource(t *testing.T) {
 		t.Fatal("two modules share an event subject, so neither owns its own name")
 	}
 }
+
+// A tool nobody serves says so at once. The difference between "that module is down" and "that
+// tool is slow" is the first thing a person asking wants, and waiting out the timeout to say it
+// is how a fast answer becomes a slow non-answer.
+func TestAskingAToolNobodyServesFailsAtOnce(t *testing.T) {
+	url := os.Getenv("MESH_TEST_NATS")
+	if url == "" {
+		t.Skip("MESH_TEST_NATS unset")
+	}
+	conn, err := nats.Connect(url)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer conn.Close()
+
+	bus := OverNATS{Conn: conn}
+	began := time.Now()
+	_, err = bus.AskTool(context.Background(), "nobody", "home", nil, 30*time.Second)
+	if err == nil {
+		t.Fatal("asking a tool nothing serves succeeded")
+	}
+	if took := time.Since(began); took > 2*time.Second {
+		t.Errorf("took %v to say nothing serves it; the caller waited out the timeout", took)
+	}
+	if !strings.Contains(err.Error(), "nothing serves") {
+		t.Errorf("the refusal does not say nobody is there: %v", err)
+	}
+}
+
+// And a served tool answers, with no reply queue to declare and no correlation to check.
+func TestAskingAServedToolAnswers(t *testing.T) {
+	url := os.Getenv("MESH_TEST_NATS")
+	if url == "" {
+		t.Skip("MESH_TEST_NATS unset")
+	}
+	conn, err := nats.Connect(url)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer conn.Close()
+
+	sub, err := conn.Subscribe(ToolSubject("shop", "price"), func(m *nats.Msg) {
+		_ = m.Respond([]byte(`{"total":12}`))
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer sub.Unsubscribe()
+
+	got, err := OverNATS{Conn: conn}.AskTool(context.Background(), "shop", "price",
+		[]byte(`{"qty":4}`), 5*time.Second)
+	if err != nil {
+		t.Fatal(err)
+	}
+	if string(got) != `{"total":12}` {
+		t.Fatalf("the answer came back as %s", got)
+	}
+}