Fix parsing of optional mode field in BigQuery Result Schema

[patricker]

closes: #26785

The code currently expects mode to be present in the returned Schema object from BigQuery, but according to the official docs, only name and type are required to be present. mode is listed as optional.

https://cloud.google.com/bigquery/docs/reference/rest/v2/tables#TableFieldSchema

I updated the code to make mode optional, with a default of NULLABLE which matches the docs:

The default value is NULLABLE.

I improved the unit tests to account for more possible scenarios.

[kevingoss2]

I am still having this issue with the latest Airflow (2.5.1) and Google providers (8.8.0).
File "/opt/bitnami/airflow/venv/lib/python3.9/site-packages/airflow/providers/common/sql/hooks/sql.py", line 349, in run
self._run_command(cur, sql_statement, parameters)
File "/opt/bitnami/airflow/venv/lib/python3.9/site-packages/airflow/providers/common/sql/hooks/sql.py", line 380, in _run_command
cur.execute(sql_statement)
File "/opt/bitnami/airflow/venv/lib/python3.9/site-packages/airflow/providers/google/cloud/hooks/bigquery.py", line 2697, in execute
description = _format_schema_for_description(query_results["schema"])
File "/opt/bitnami/airflow/venv/lib/python3.9/site-packages/airflow/providers/google/cloud/hooks/bigquery.py", line 3006, in _format_schema_for_description
field["mode"] == "NULLABLE",
KeyError: 'mode'

[patricker]

@kevingoss2 All I can think is that you somehow aren't actually using Google Providers 8.8.0? Looking at the code for the 8.8.0 Tag, the line numbers/code in the error don't line up with that version at all: https://github.com/apache/airflow/blob/providers-google/8.8.0/airflow/providers/google/cloud/hooks/bigquery.py#L2992

[kevingoss2]

@patricker You are correct. That version of the code does not have the error. The issue is that when I do a pip freeze on the docker container it shows 8.8.0. Will try to see what is going on. Thanks for the response!

Note: I figured out why my dependency was getting the wrong version and all is working now. Thanks.