impl(v3): Multiplexed sessions always enabled #9265

Job	Run time
debug_job	4s
Save PR ref	4s
Windows-CMake / cmake + msvc-2022 + x64 + Release + Core1	4m 59s
macOS-CMake	0s
Windows-Bazel	0s
macOS-Bazel	0s
	5m 7s

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