feat(huma): error formatter matching legacy Vikunja JSON shape

pkg/routes/api/v1/humaapi/errors.go

@@ -0,0 +1,29 @@
+package humaapi
+
+import (
+	"github.com/danielgtaylor/huma/v2"
+)
+
+// vikunjaError is the JSON shape Vikunja's existing error_handler.go emits
+// for string-style errors. We preserve it so frontend contracts don't change.
+type vikunjaError struct {
+	StatusCode int    `json:"-"`
+	Code       int    `json:"code,omitempty"`
+	Message    string `json:"message"`
+}
+
+func (e *vikunjaError) Error() string  { return e.Message }
+func (e *vikunjaError) GetStatus() int { return e.StatusCode }
+
+// NewVikunjaError produces an error that serializes to Vikunja's legacy shape
+// (`{"message": "..."}` for plain errors; `{"code": X, "message": "..."}` when
+// a domain code is supplied). Registered as huma.NewError so every Huma
+// handler's error return routes through here.
+func NewVikunjaError(status int, msg string, errs ...error) huma.StatusError {
+	return &vikunjaError{StatusCode: status, Message: msg}
+}
+
+// Install replaces huma.NewError globally. Call once at init.
+func Install() {
+	huma.NewError = NewVikunjaError
+}

pkg/routes/api/v1/humaapi/errors_test.go

@@ -0,0 +1,31 @@
+package humaapi
+
+import (
+	"encoding/json"
+	"net/http"
+	"testing"
+
+	"github.com/danielgtaylor/huma/v2"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func TestVikunjaErrorShape_BasicCodeMessage(t *testing.T) {
+	err := NewVikunjaError(http.StatusForbidden, "Forbidden")
+	b, marshalErr := json.Marshal(err)
+	require.NoError(t, marshalErr)
+
+	var got map[string]any
+	require.NoError(t, json.Unmarshal(b, &got))
+	assert.Equal(t, "Forbidden", got["message"])
+	// must not include RFC 9457 fields
+	_, hasType := got["type"]
+	_, hasTitle := got["title"]
+	assert.False(t, hasType, "unexpected RFC 9457 field 'type'")
+	assert.False(t, hasTitle, "unexpected RFC 9457 field 'title'")
+}
+
+func TestVikunjaErrorShape_StatusCoderInterface(t *testing.T) {
+	var e huma.StatusError = NewVikunjaError(http.StatusNotFound, "not found")
+	assert.Equal(t, http.StatusNotFound, e.GetStatus())
+}