fix: Service IPC sockets during dag bundle refresh to prevent child process blocking

[rahulchheda]

What

Service IPC sockets in a background thread during _refresh_dag_bundles() so child parsing processes don't block on Variable.get() / Connection.get() calls.

Why

In _run_parsing_loop(), IPC is only serviced once per iteration via _service_processor_sockets(). While the main loop executes _refresh_dag_bundles() and other operations, child processes that call Variable.get() block on socket.recv() waiting for the parent to respond. With many DAG files and multiple parsing processes, this turns sub-second parses into multi-minute waits.

This is a regression from Airflow 2.x where Variable.get() was a direct DB query with no IPC dependency.

For comparison, WatchedSubprocess._monitor_subprocess() in the task execution path has a tight loop that continuously services IPC — the dag-processor lacks this.

How

• Added a _ipc_service_thread() context manager that runs _service_processor_sockets() in a daemon thread with a 100ms poll interval
• Wrapped _refresh_dag_bundles() with this context manager — the longest-running phase where child processes are likely to be blocked
• Thread is automatically stopped when the context exits, before the main loop continues with its own _service_processor_sockets() call

The change is minimal and non-invasive — existing IPC servicing in the main loop is untouched.

Closes #65369

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