stream: avoid duplicate writes in toWritable #6800

Job	Run time
Build slim tarball	1m 16s
x86_64-linux: with shared libraries	29m 32s
aarch64-linux: Cache V8 build	22s
x86_64-darwin: with shared libraries	44m 1s
aarch64-darwin: with shared libraries	24m 40s
aarch64-linux: with shared openssl-1.1.1w	25m 4s
aarch64-linux: with shared boringssl-0.20260413.0	24m 0s
aarch64-linux: with shared openssl-3.6.1	24m 29s
aarch64-linux: with shared openssl-4.0.0	24m 52s
aarch64-linux: with shared openssl-3.0.19	24m 4s
aarch64-linux: with shared openssl-3.5.5	24m 20s
	4h 6m 40s

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