stream: fix merge() idle source draining #7305

Job	Run time
Build slim tarball	1m 26s
aarch64-linux: Cache V8 build	21s
x86_64-linux: with shared libraries	26m 17s
x86_64-darwin: with shared libraries	53m 14s
aarch64-darwin: with shared libraries	23m 59s
aarch64-linux: with shared openssl-4.0.0	24m 30s
aarch64-linux: with shared openssl-3.0.20	25m 5s
aarch64-linux: with shared openssl-3.6.2	23m 58s
aarch64-linux: with shared boringssl-0.20260508.0	25m 35s
aarch64-linux: with shared openssl-1.1.1w	23m 45s
aarch64-linux: with shared openssl-3.5.6	24m 11s
	4h 12m 21s

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