stream: preserve toReadableSync batch after backpressure #6644

Job	Run time
Build slim tarball	1m 52s
aarch64-linux: Cache V8 build	23s
x86_64-darwin: with shared libraries	39m 23s
x86_64-linux: with shared libraries	26m 31s
aarch64-darwin: with shared libraries	26m 16s
aarch64-linux: with shared openssl-4.0.0	23m 4s
aarch64-linux: with shared openssl-3.5.5	22m 32s
aarch64-linux: with shared openssl-1.1.1w	22m 40s
aarch64-linux: with shared openssl-3.0.19	22m 32s
aarch64-linux: with shared boringssl-0.20260413.0	23m 12s
aarch64-linux: with shared openssl-3.6.1	22m 33s
	3h 50m 58s

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