Answer a panicking verb with an error, say flag errors in the answer, and leave refused logins out of D15

Read verbs now run in the serving process, where a panic would end every
call; refused logins are nobody's reconnect loop (review of hq issue 327).

internal/link/calls_test.go

@@ -11,6 +11,10 @@ import (
 	"sync"
 	"testing"
 	"time"
+
+	"github.com/nats-io/nats.go"
+
+	"github.com/novox/mesh-controller/internal/testbus"
 )
 
 // answers collects what a call answered, and fails a second answer: the bus permits one.
@@ -223,3 +227,29 @@ func TestAJoinTokenIsShownToItsCallerAndNotKept(t *testing.T) {
 		}
 	}
 }
+
+// A verb whose handler panics answers an error, and the controller serves the next call (review of
+// novox/hq issue 327: read verbs are answered in the serving process now).
+func TestAHandlerThatPanicsAnswersAnError(t *testing.T) {
+	conn, err := nats.Connect(testbus.URL(t))
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer conn.Close()
+	stop, err := OverNATS{Conn: conn}.ServeSeatTools("panicky", map[string]ToolHandler{
+		"boom": func(context.Context, json.RawMessage) (any, error) { panic("nil map") },
+		"fine": func(context.Context, json.RawMessage) (any, error) { return "ok", nil },
+	}, nil)
+	if err != nil {
+		t.Fatal(err)
+	}
+	defer stop()
+	answer, err := AskMeshSeatTool(context.Background(), conn, "panicky", "boom", map[string]any{}, 5*time.Second)
+	if err != nil || !strings.Contains(answer.Error, "failed inside the controller") {
+		t.Fatalf("a panic answered %+v (%v)", answer, err)
+	}
+	answer, err = AskMeshSeatTool(context.Background(), conn, "panicky", "fine", map[string]any{}, 5*time.Second)
+	if err != nil || answer.Error != "" {
+		t.Fatalf("the call after a panic answered %+v (%v)", answer, err)
+	}
+}