Add backend capacity policy configuration

public_contract_test.go

@@ -303,6 +303,28 @@ func TestBackendOptionsAccumulateAndRegistrationsAreEngineLocal(t *testing.T) {
 	}
 }
 
+func TestWithBackendCopiesQueueCapacity(t *testing.T) {
+	queueCapacity := 4
+	option := promptkit.WithBackend(promptkit.Backend{
+		ID:               "custom",
+		Endpoint:         "http://custom.example/v1",
+		ConcurrencyLimit: 1,
+		QueueCapacity:    &queueCapacity,
+	})
+	queueCapacity = -1
+
+	_, err := promptkit.NewEngine(promptkit.Config{},
+		promptkit.WithPromptFS(contractPromptFS("prompt", "profile", "message"), "."),
+		option,
+		promptkit.WithProfiles(promptkit.Profile{
+			ID: "profile", BackendID: "custom", Model: "model",
+		}),
+	)
+	if err != nil {
+		t.Fatalf("construct engine after mutating queue pointer: %v", err)
+	}
+}
+
 func TestBackendRegistrationRejectsInvalidAndDuplicateDefinitions(t *testing.T) {
 	cycle := map[string]any{}
 	cycle["self"] = cycle