fix(bigquery): truncate when casting float to int

BREAKING CHANGE: when using the bigquery backend, casting float to int
will no longer round floats to the nearest integer

ibis/backends/bigquery/registry.py

@@ -56,6 +56,12 @@ def bigquery_cast_interval_to_integer(compiled_arg, from_, to):
     return f"EXTRACT({from_.resolution.upper()} from {compiled_arg})"
 
 
+@bigquery_cast.register(str, dt.Floating, dt.Integer)
+def bigquery_cast_floating_to_integer(compiled_arg, from_, to):
+    """Convert FLOAT64 to INT64 without rounding."""
+    return f"CAST(TRUNC({compiled_arg}) AS INT64)"
+
+
 @bigquery_cast.register(str, dt.DataType, dt.DataType)
 def bigquery_cast_generate(compiled_arg, from_, to):
     """Cast to desired type."""

ibis/backends/bigquery/tests/system/test_client.py

@@ -96,6 +96,12 @@ def test_cast_string_to_date(alltypes, df):
     tm.assert_series_equal(result, expected)
 
 
+def test_cast_float_to_int(alltypes, df):
+    result = (alltypes.float_col - 2.55).cast("int64").to_pandas().sort_values()
+    expected = (df.float_col - 2.55).astype("int64").sort_values()
+    tm.assert_series_equal(result, expected, check_names=False)
+
+
 def test_has_partitions(alltypes, parted_alltypes, con):
     col = con.partition_column
     assert col not in alltypes.columns