fix: not bail! on kinesis's state if scan.startup.mode set to timestamp

[tabVersion]

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What's changed and what's your intention?

introduced in #12048

the create split gonna fail

• if user set scan.startup.mode to timestamp
• a new shard is discovered and ready to consume

-> lead to create source task failure

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[tabVersion]

@stdrc talked about enumerator should inherent offset settings from source props. But it cannot handle the case if the user set scan.startup.mode to latest, leading to data loss from the shard starts to the time we actually start reading data.

[tabVersion]

#18040
the pr labeled with cherry-pick-release-1.10 but actually picked to release-1.9 🤣