fix: localize toml parse errors

internal/agents/writeback_test.go

@@ -21,6 +21,7 @@ func TestValidateInvalidTOMLReturnsInvalidAndDoesNotWrite(t *testing.T) {
 	if len(got.Errors) == 0 {
 		t.Fatalf("expected validation errors: %#v", got)
 	}
+	assertChineseTOMLError(t, got.Errors[0])
 	assertFileContent(t, target, `name = "旧名称"`+"\n")
 }
 
@@ -56,6 +57,7 @@ func TestWriteInvalidTOMLRejectsAndLeavesOriginal(t *testing.T) {
 	if err == nil {
 		t.Fatal("expected invalid TOML write to be rejected")
 	}
+	assertChineseTOMLError(t, err.Error())
 	assertFileContent(t, target, `name = "旧名称"`+"\n")
 }
 
@@ -159,3 +161,13 @@ func assertFileContent(t *testing.T, path string, want string) {
 		t.Fatalf("%s content = %q, want %q", path, string(got), want)
 	}
 }
+
+func assertChineseTOMLError(t *testing.T, message string) {
+	t.Helper()
+	if strings.Contains(message, "invalid syntax") {
+		t.Fatalf("TOML error leaked English parser text: %q", message)
+	}
+	if !strings.Contains(message, "字符串字段语法无效") {
+		t.Fatalf("TOML error = %q, want Chinese string syntax error", message)
+	}
+}