stream: fix merge() idle source draining #7246

Job	Run time
Build slim tarball	1m 13s
aarch64-linux: Cache V8 build	22s
aarch64-darwin: with shared libraries	22m 37s
x86_64-darwin: with shared libraries	35m 55s
x86_64-linux: with shared libraries	23m 54s
aarch64-linux: with shared openssl-4.0.0	23m 50s
aarch64-linux: with shared openssl-3.6.1	24m 23s
aarch64-linux: with shared openssl-1.1.1w	24m 6s
aarch64-linux: with shared boringssl-0.20260413.0	23m 25s
aarch64-linux: with shared openssl-3.5.5	24m 21s
aarch64-linux: with shared openssl-3.0.19	23m 48s
	3h 47m 54s

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