fix(starrocks): fix starrocks coordinator incorrect try_join_all on join handle

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What's changed and what's your intention?

Previously in #16816, we used a try_join_all directly on the JoinHandle, which only returns the JoinError, while the error in the output of JoinHandle is ignored, because it is treated as Ok(Err(...)).

In this PR, we transform the future of JoinHandle to a single layer result, so that the error in join handle output will be treated as error.

Checklist

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risingwavelabs / risingwave