fix(duckdb): use to_arrow_table() instead of deprecated fetch_arrow_table()

[johannesboyne]

What

Route the five DuckDB-backend arrow extractions (get_schema, list_catalogs, list_databases, _metadata, _get_schema_using_query) through a small _to_pyarrow_table() helper.

Why

DuckDB 1.5 deprecated DuckDBPyRelation/DuckDBPyConnection.fetch_arrow_table() in favour of to_arrow_table(), emitting a DeprecationWarning on every call (this floods the dq-platform qserviceworker logs).

A plain rename to to_arrow_table() is not safe: ibis supports duckdb>=0.10.3, and on those older versions to_arrow_table does not exist on the connection/cursor object — a rename produces 218 backend test failures (AttributeError: 'DuckDBPyConnection' object has no attribute 'to_arrow_table').

The helper prefers to_arrow_table() when present (new duckdb → no warning) and falls back to fetch_arrow_table() otherwise (old duckdb → still works, not yet deprecated there):

def _to_pyarrow_table(result):
    to_arrow_table = getattr(result, "to_arrow_table", None)
    if to_arrow_table is not None:
        return to_arrow_table()
    return result.fetch_arrow_table()

Notes

• Return type (pyarrow.Table) and all downstream usage unchanged; verified both helper branches and that the DeprecationWarning is gone on duckdb 1.5.1.
• Branched off the 12.0.0 release commit (01e429b) that dq-platform pins, so it carries no unrelated changes.

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