Categorical revmaps are not merged when concatenated inside a struct

[orlp]

Reproduction:

>>> a = pl.DataFrame({"x": ["a"]}, schema={"x": pl.Categorical})
>>> b = pl.DataFrame({"x": ["b"]}, schema={"x": pl.Categorical})
>>> pl.concat([a.select(pl.struct(pl.col.x)), b.select(pl.struct(pl.col.x))])
shape: (2, 1)
┌───────────┐
│ x         │
│ ---       │
│ struct[1] │
╞═══════════╡
│ {"a"}     │
│ {"a"}     │
└───────────┘

This should obviously be {"a"}, {"b"}.

[coastalwhite]

This was fixed by #18815.