fix(metadata v2): existing actor splits were not updated

[fuyufjh]

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What's changed and what's your intention?

As title.

This is the root cause of duplicated source splits. Resolves #16235.

1. Assume there is only 1 source split and 4 source actors at the beginning.
2. Run alter materialized view ... set parallelism = 8. There is 50% chance that the only 1 split is assigned to a new actor.
3. Because of this bug, the old actor's split assignment was not updated, causing duplication.
4. Once recovery or restarts, the duplicated splits will cause duplicated state table writes, causing assertion failure of bug: dedicated cdc source writes the same full key more than once in an epoch #16235.

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[shanicky]

Lgtm! Thanks!

[xxchan]

LGTM

Ideally we can add some assertions (maybe after scaling/after recovery) to ensure actor_splits is consistent.

And in the future we can avoid persisting actor_splits to reduce complexity and inconsistencies.