Fix RealtimeBox broken API + `realtime_box_best_effort.h` proper deprecation #480

Job	Run time
binary (humble, main) / humble main	3m 14s
binary (humble, testing) / humble testing	3m 29s
binary (jazzy, main) / jazzy main	3m 9s
binary (jazzy, testing) / jazzy testing	3m 23s
binary (rolling, main) / rolling main	3m 14s
binary (rolling, testing) / rolling testing	3m 14s
	19m 43s

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