fix(cdc source): fix a possible cause of cdc offset rewind

[stdrc]

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What's changed and what's your intention?

In current impl, we split cdc heartbeat events and data events into separate stream chunks, and emit heartbeat chunk first, so that the last-win updates of CdcSplits will be won by data chunks if any. This PR fix a corner case where heartbeat chunk is not emitted before data chunk.

This corner case may be a possible cause of #15312.

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[StrikeW]

This corner case may be a possible cause of #15312.

Could you elaborate your thought in the issue thread?

[stdrc]

This corner case may be a possible cause of #15312.

Could you elaborate your thought in the issue thread?

Sure, #15312 (comment)

[StrikeW]

LGTM

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