Chunk labeled with tbl- which outputs a gt table with a tab_spanner generates an invalid docx file

[raffaem]

Prework

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Description

This is a spin-off of #1898 focused on chunks labeled as a table.

If you write a chunk labeled as a table for cross-references purpose (that is, #| label: tbl-SOMETHING), and you output a gt table with a tab spanner, the resulting docx file is invalid.

It won't even open in Microsoft Word for Windows, it will say the document contains unreadable content:

Reproducible example

---
title: "test"
format: docx
---

```{r}
#| label: tbl-reg
#| echo: false
#| output: true
library(gt)
tbl_comp_names <- structure(list(lvl = c("BEBRAS", "Advent of code"), 
                                 per = c(79.5,
                                         37.2), 
                                 per_grand = c(57.4, 26.9), 
                                 grandtotal = c(108L, 108L), 
                                 total = c(78L, 78L)), 
                            row.names = c(NA, -2L), 
                            class = c("tbl_df", 
                                      "tbl", "data.frame"))

tbl_comp_names %>% 
  gt() %>%
  tab_spanner(
    columns = starts_with("per"),
    label=("percentage")) %>%
  tab_spanner(
    columns = ends_with("total"),
    label=("n"))

Expected result

The docx file should be valid

Session info

> sessionInfo()
R version 4.4.2 (2024-10-31)
Platform: x86_64-pc-linux-gnu
Running under: Arch Linux

Matrix products: default
BLAS:   /usr/lib/libblas.so.3.12.0 
LAPACK: /usr/lib/liblapack.so.3.12.0

locale:
 [1] LC_CTYPE=en_US.UTF-8       LC_NUMERIC=C               LC_TIME=en_DK.UTF-8       
 [4] LC_COLLATE=en_US.UTF-8     LC_MONETARY=en_US.UTF-8    LC_MESSAGES=en_US.UTF-8   
 [7] LC_PAPER=en_US.UTF-8       LC_NAME=C                  LC_ADDRESS=C              
[10] LC_TELEPHONE=C             LC_MEASUREMENT=en_US.UTF-8 LC_IDENTIFICATION=C       

time zone: Europe/Rome
tzcode source: system (glibc)

attached base packages:
[1] stats     graphics  grDevices utils     datasets  methods   base     

other attached packages:
[1] gt_0.11.1

loaded via a namespace (and not attached):
 [1] digest_0.6.37     R6_2.5.1          fastmap_1.2.0     tidyselect_1.2.1  xfun_0.49        
 [6] magrittr_2.0.3    glue_1.8.0        tibble_3.2.1      knitr_1.49        parallel_4.4.2   
[11] htmltools_0.5.8.1 pkgconfig_2.0.3   dplyr_1.1.4       generics_0.1.3    lifecycle_1.0.4  
[16] xml2_1.3.6        cli_3.6.3         sass_0.4.9        vctrs_0.6.5       withr_3.0.2      
[21] compiler_4.4.2    rstudioapi_0.17.1 tools_4.4.2       evaluate_1.0.1    pillar_1.10.0    
[26] rlang_1.1.4