Harden local runtime safety and error handling

internal/handlers/respond_test.go

@@ -0,0 +1,41 @@
+package handlers
+
+import (
+	"encoding/json"
+	"testing"
+)
+
+func TestClientFacingErrorMessageKeepsDomain4xx(t *testing.T) {
+	t.Parallel()
+
+	got := clientFacingErrorMessage(400, "invalid request", &json.SyntaxError{Offset: 1})
+	if got != "invalid request" {
+		t.Fatalf("expected fallback for decode error, got %q", got)
+	}
+}
+
+func TestClientFacingErrorMessagePreservesBusinessMessage(t *testing.T) {
+	t.Parallel()
+
+	err := errString("main project variant cannot be deleted")
+	got := clientFacingErrorMessage(400, "invalid request", err)
+	if got != err.Error() {
+		t.Fatalf("expected business message, got %q", got)
+	}
+}
+
+func TestClientFacingErrorMessageHidesTechnical4xx(t *testing.T) {
+	t.Parallel()
+
+	err := errString("sql: no rows in result set")
+	got := clientFacingErrorMessage(404, "resource not found", err)
+	if got != "resource not found" {
+		t.Fatalf("expected fallback for technical error, got %q", got)
+	}
+}
+
+type errString string
+
+func (e errString) Error() string {
+	return string(e)
+}