Description

This pull request fixes a bug where read_parquet_votable incorrectly emits AstropyUserWarning messages for every string column when reading VOParquet files produced by non-astropy writers (e.g. TAP services).

read_parquet_votable delegates the raw data load to read_table_parquet via Table.read(filename, format="parquet"). That code path looks for table::len::<name> keys in the Parquet file metadata to determine string column widths. Those keys are an astropy-private convention written only by write_parquet_table, but not part of the IVOA VOTable-in-Parquet specification.

So conformant VOParquet files not produced by astropy will not include these keys, leading to warnings incorrectly being fired for any such files.

The fix here adds a string_lengths parameter to read_table_parquet that lets callers supply column widths directly, bypassing the table::len lookup and the associated scans & warnings.

read_parquet_votable uses this parameter by extracting the widths from the arraysize attributes of the <FIELD> elements in the embedded VOTable XML, as that information should always be present in every conformant VOParquet file.

Fixes #19783

Changes

astropy/io/misc/parquet.py

• read_table_parquet: new string_lengths: dict[str, int | None] | None parameter. When a column name is present in the dict, the value is used as the string width (None means variable-length). The table::len metadata lookup and scan/warning are only reached when the column is absent from string_lengths.
• read_parquet_votable: parses the embedded VOTable XML, builds a complete string_lengths dict from <FIELD arraysize="..."> attributes, and passes it to the Table.read(filename, format="parquet") call.

astropy/io/misc/tests/test_parquet_votable.py

• Added _write_voparquet helper that writes a minimal VOParquet file using
  pyarrow directly, simulating a TAP service (no table::len written).
• New tests covering various cases.
• Updated test_compare_parquet_votable to assert that VOParquet reads never
  emit the warning (plain parquet reads still do).