stream: validate broadcast writer writev chunks #6778

Job	Run time
Build slim tarball	2m 33s
aarch64-linux: Cache V8 build	19s
x86_64-darwin: with shared libraries	43m 11s
x86_64-linux: with shared libraries	24m 55s
aarch64-darwin: with shared libraries	22m 49s
aarch64-linux: with shared openssl-3.0.19	24m 24s
aarch64-linux: with shared openssl-1.1.1w	22m 48s
aarch64-linux: with shared openssl-4.0.0	23m 32s
aarch64-linux: with shared openssl-3.5.5	22m 59s
aarch64-linux: with shared openssl-3.6.1	23m 0s
aarch64-linux: with shared boringssl-0.20260413.0	22m 45s
	3h 53m 15s

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