concat_on_disk with join='outer' doesn't retain all .obsm fields #2394

# /// script
# requires-python = ">=3.12"
# dependencies = [
#   "anndata@git+https://github.com/scverse/anndata.git",
# ]
# ///

import anndata
import numpy as np
a = ad.AnnData(
        X=np.ones((2, 10)),
        obsm={'X_emb': np.arange(4).astype(np.float64).reshape((2, 2))},
)
b = ad.AnnData(
        X=np.ones((4, 12)),
)

c = ad.concat([a, b], join="outer")
assert 'X_emb' in c.obsm.keys()

a.write_h5ad('a.h5ad')
b.write_h5ad('b.h5ad')

ad.experimental.concat_on_disk(['a.h5ad', 'b.h5ad'], out_file='d.h5ad', join="outer")

d = ad.read_h5ad('d.h5ad')
assert 'X_emb' in d.obsm.keys()