refactor backend API structure

internal/httpapi/openai/error_shape_test.go

@@ -0,0 +1,34 @@
+package openai
+
+import (
+	"encoding/json"
+	"net/http"
+	"net/http/httptest"
+	"testing"
+)
+
+func TestWriteOpenAIErrorIncludesUnifiedFields(t *testing.T) {
+	rec := httptest.NewRecorder()
+	writeOpenAIError(rec, http.StatusBadRequest, "invalid input")
+	if rec.Code != http.StatusBadRequest {
+		t.Fatalf("expected 400, got %d", rec.Code)
+	}
+
+	var body map[string]any
+	if err := json.Unmarshal(rec.Body.Bytes(), &body); err != nil {
+		t.Fatalf("decode body: %v", err)
+	}
+	errObj, _ := body["error"].(map[string]any)
+	if errObj["message"] != "invalid input" {
+		t.Fatalf("unexpected message: %v", errObj["message"])
+	}
+	if errObj["type"] != "invalid_request_error" {
+		t.Fatalf("unexpected type: %v", errObj["type"])
+	}
+	if errObj["code"] != "invalid_request" {
+		t.Fatalf("unexpected code: %v", errObj["code"])
+	}
+	if _, ok := errObj["param"]; !ok {
+		t.Fatal("expected param field")
+	}
+}