fix: disable sink decouple for trivial sink

[hzxa21]

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What's changed and what's your intention?

Sink decoupling is not applicable to trivial sink because it introduces overhead with no benefits.

Sink decoupling will also make "Write Ops" > 0 even if source throughput drops to 0 because sink log store will write one row per barrier regardless of whether the barrier is empty, which may confuse the perf pipeline.

Related: #18181, #18268

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