fix(stream): fix functional dependencies for row merge

[kwannoel]

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What's changed and what's your intention?

Add test case to demo the fix. Basically it will panic because the functional dependency is wrongly derived. The output size of the fd did not match the expected input size to the project plan.

That's because the fd previously is just a clone of the lhs. This PR fixes it.

Checklist

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[stdrc]

rubber stamp