fix(stream): handle empty upstream inputs in Merge executor gracefully

[silver-ymz]

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What's changed and what's your intention?

fix failed CI https://buildkite.com/risingwavelabs/pull-request/builds/88963#019b9326-f62f-451a-ac2d-711b9aa350f2

It seems that it's a regression introduced in #24289. I'm not sure about it.

Checklist

• I have written necessary rustdoc comments.
• I have added necessary unit tests and integration tests.
• I have added test labels as necessary.
• I have added fuzzing tests or opened an issue to track them.
• My PR contains breaking changes.
• My PR changes performance-critical code, so I will run (micro) benchmarks and present the results.
• I have checked the Release Timeline and Currently Supported Versions to determine which release branches I need to cherry-pick this PR into.

Documentation

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Release note

[Copilot]

Pull request overview

This PR fixes a regression introduced in #24289 where the Merge executor's BufferChunks implementation would panic when all upstream inputs closed unexpectedly. The fix replaces an unreachable!() assertion with proper error handling to enable graceful recovery.

Key Changes:

• Replaced unreachable!() panic with StreamExecutorError::channel_closed() when upstream inputs are closed
• Added explanatory comment describing when this scenario can occur (e.g., during exchange channel closure during recovery)

[BugenZhao]

Is this the same issue as #24336?

Perhaps we should identify the root cause and fix it instead.
Some more discussions here: https://risingwave-labs.slack.com/archives/C094S2N254M/p1767691876158269