impl(v3): Multiplexed sessions always enabled (#15941) #9283

Job	Run time
debug_job	3s
Save PR ref	3s
Windows-Bazel / bazel + msvc-2022 + opt + Core	2m 56s
Windows-CMake	0s
macOS-CMake	0s
macOS-Bazel	-1s
Notify-Google-Chat	3s
	3m 4s

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