stream: fix merge() idle source draining #7510

Job	Run time
Build slim tarball	1m 9s
x86_64-linux: with shared libraries	27m 7s
aarch64-linux: Cache V8 build	20s
aarch64-darwin: with shared libraries	27m 4s
x86_64-darwin: with shared libraries	43m 14s
aarch64-linux: with shared boringssl-0.20260508.0	26m 25s
aarch64-linux: with shared openssl-3.6.2	25m 56s
aarch64-linux: with shared openssl-4.0.0	25m 35s
aarch64-linux: with shared openssl-3.5.6	25m 53s
aarch64-linux: with shared openssl-3.0.20	25m 23s
aarch64-linux: with shared openssl-1.1.1w	25m 12s
	4h 13m 18s

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