chore: Migrate coalesce_op operator to SQLGlot

[jialuoo]

Migrated the coalesce_op operator from Ibis to SQLGlot.

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Fixes b/447388852 🦕

[jialuoo]

The ibis sql:

SELECT
  `t0`.`int64_col`,
  COALESCE(`t0`.`int64_too`, `t0`.`int64_col`) AS `int64_too`
FROM (
  SELECT
    `int64_col`,
    `int64_too`
  FROM `bigframes-dev.sqlglot_test.scalar_types`
) AS `t0`

[chelsea-lin]

Can you please enable the existing engine test test_engines_coalesce_op by adding "bq-sqlglot" to the "engine" parameters.

python-bigquery-dataframes/tests/system/small/engines/test_generic_ops.py

Line 332 in 0a3e172

@pytest.mark.parametrize("engine", ["polars", "bq"], indirect=True)

[jialuoo]

Nice catch, thanks.