Default sending_queue.batch.sizer to items when sending_queue.sizer is not set #32319

Job	Run time
windows-unittest (windows-11-arm)	11m 34s
windows-service-test (windows-2022)	3m 12s
windows-unittest (windows-2025)	18m 12s
windows-service-test (windows-2025)	3m 32s
windows-unittest (windows-2022)	15m 46s
windows-service-test (windows-11-arm)	3m 26s
	55m 42s

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