Fix passing labels from Airflow connections to BQ jobs

[VladaZakharova]

The current documentation on BigQuery suggests that the logic of adding labels that were set in the Airflow connection should be also expected in the labels section of the created Big Query Job:
https://airflow.apache.org/docs/apache-airflow-providers-google/stable/connections/bigquery.html

So this current flow looks like a bug, that should be fixed.

This PR merges connection-level labels into the job configuration before the job is submitted.
The new behavior is:

If the operator config has no labels, it copies labels from the connection.
If the operator config already has labels, it merges them.
If the same label key exists in both places, the operator config wins.
If the operator explicitly sets labels=None, that stays unchanged.

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[boring-cyborg]

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