Add local backend convenience constructor

public_contract_test.go

@@ -5,6 +5,7 @@ import (
 	"encoding/json"
 	"errors"
 	"fmt"
+	"reflect"
 	"strings"
 	"sync"
 	"sync/atomic"
@@ -138,6 +139,42 @@ func TestUnknownProfileBackendHasProfileLoadIdentity(t *testing.T) {
 	}
 }
 
+func TestLocalBackendConstructsAndRegistersConventionalBackend(t *testing.T) {
+	const limit = 2
+	endpoint := "http://local.example/v1"
+	backend := promptkit.LocalBackend(endpoint, limit)
+	want := promptkit.Backend{
+		ID:               promptkit.BackendLocal,
+		Endpoint:         endpoint,
+		ConcurrencyLimit: limit,
+	}
+	if !reflect.DeepEqual(backend, want) {
+		t.Fatalf("LocalBackend()=%+v, want %+v", backend, want)
+	}
+
+	engine, err := promptkit.NewEngine(promptkit.Config{},
+		promptkit.WithPromptFS(contractPromptFS("prompt", "local-profile", "message"), "."),
+		promptkit.WithBackend(backend),
+		promptkit.WithProfiles(promptkit.Profile{
+			ID:        "local-profile",
+			BackendID: promptkit.BackendLocal,
+			Model:     "model",
+		}),
+	)
+	if err != nil {
+		t.Fatalf("construct engine with local backend: %v", err)
+	}
+
+	prepared, err := engine.Prepare(context.Background(), promptkit.RunRequest{PromptID: "prompt"})
+	if err != nil {
+		t.Fatalf("prepare with local backend: %v", err)
+	}
+	if prepared.SelectedBackendID != promptkit.BackendLocal ||
+		prepared.EffectiveModelParams.Endpoint != endpoint {
+		t.Fatalf("unexpected local backend preparation: %+v", prepared)
+	}
+}
+
 func TestCustomBackendFlowsThroughProfilesOverridesAndInjectedClient(t *testing.T) {
 	t.Setenv("CUSTOM_LLM_KEY", "test-key")
 	client := &fakeLLMClient{response: &promptkit.GenerateResponse{Content: "ok"}}