bug: in bigquery to_polars doesn't respect timestamp units

[aandrestrumid]

What happened?

In bigquery timestamp and datetime have got microseconds precision (us).

When executing a query with to_pyarrow it is accurately reflected in the resulting schema, using microseconds.

When using to_polars, I get timestamp columns with nanosecond precision. It should be microseconds.

import ibis
import polars as pl
import pyarrow as pa

QUERY = 'SELECT TIMESTAMP "2025-01-01 12:34:56" AS ts'

connection = ibis.bigquery.connect()
query = connection.sql(QUERY)

arrow_table = query.to_pyarrow()
assert arrow_table["ts"].type == pa.timestamp("us", "UTC")
from_arrow = pl.from_arrow(arrow_table)
assert from_arrow["ts"].dtype == pl.Datetime("us", "UTC")

direct_from_ibis = query.to_polars()
assert direct_from_ibis['ts'].dtype == pl.Datetime("ns", "UTC") # should be us

What version of ibis are you using?

11.0.0

What backend(s) are you using, if any?

bigquery

Relevant log output

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