Add CeleryExecutor callback workload routing test

[leeyspaul]

Adds focused CeleryExecutor unit coverage for routing ExecuteCallback workloads through _process_workloads().

This verifies that callback workloads are sent to Celery with:

• the callback ID as the workload key
• the default queue when no callback queue is provided
• the callback-provided queue when present

This is intended to strengthen executor compatibility coverage for the Airflow 3.2+ callback workload path.

Related: #65793

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[SameerMesiah97]

Approved pending green CI.