Fix comma-separated selected_fields in BigQuery to SQL transfers

[FrankYang0529]

Why

• The docstring says selected_fields is a "List of fields to return (comma-separated)". The operator passes that value to two places: BigQueryHook.list_rows() splits it on commas, but insert_rows(target_fields=...) does not.

• common.sql builds the column list with ", ".join(map(escape_word, target_fields)), so a string is iterated character by character. With selected_fields="emp_name,salary":

  -- before
  INSERT INTO employees (e, m, p, _, n, a, m, e, ",", s, a, l, a, r, y) VALUES (%s, %s)
  -- after
  INSERT INTO employees (emp_name, salary) VALUES (%s, %s)

How

• BigQueryToSqlBaseOperator.__init__ normalises a comma-separated string into list[str], so BigQueryToPostgres / BigQueryToMySql / BigQueryToMsSql all get a real column list on both the BigQuery read and the SQL write side.

Verification

• uv run --project providers/google pytest providers/google/tests/unit/google/cloud/transfers/test_bigquery_to_postgres.py providers/google/tests/unit/google/cloud/transfers/test_bigquery_to_mysql.py -v

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Was generative AI tooling used to co-author this PR?

• Yes - Claude Code

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