Flush statistics during idle periods in parallel apply worker.

Flush statistics during idle periods in parallel apply worker.

Parallel apply workers previously failed to report statistics while
waiting for new work in the main loop. This resulted in the stats from the
most recent transaction remaining unbuffered, leading to arbitrary
reporting delays—particularly when streamed transactions were infrequent.

This commit ensures that statistics are explicitly flushed when the worker
is idle, providing timely visibility into accumulated worker activity.

Author: Zhijie Hou <houzj.fnst@fujitsu.com>
Reviewed-by: Chao Li <li.evan.chao@gmail.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>
Backpatch-through: 16, where it was introduced
Discussion: https://postgr.es/m/TYRPR01MB1419579F217CC4332B615589594202@TYRPR01MB14195.jpnprd01.prod.outlook.com