stream: avoid duplicate writes in toWritable #6850

Job	Run time
Build slim tarball	2m 19s
x86_64-darwin: with shared libraries	39m 9s
aarch64-darwin: with shared libraries	27m 21s
x86_64-linux: with shared libraries	28m 59s
aarch64-linux: Cache V8 build	25s
aarch64-linux: with shared boringssl-0.20260413.0	24m 36s
aarch64-linux: with shared openssl-3.0.19	25m 14s
aarch64-linux: with shared openssl-3.5.5	24m 52s
aarch64-linux: with shared openssl-3.6.1	23m 59s
aarch64-linux: with shared openssl-1.1.1w	24m 32s
aarch64-linux: with shared openssl-4.0.0	25m 3s
	4h 6m 29s

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