[Fix][Core] Use asyncio.Event instead of AsyncioActorExit exception to terminate async actor

[MortalHappiness]

Why are these changes needed?

When an asyncio task creates another asyncio task, raising AsyncioActorExit cannot make the caller exit because they are not the same task. Therefore, this PR removes the AsyncioActorExit custom exception and uses asyncio.Event for synchronization among multiple asyncio tasks. This ensures that even if the user creates a different asyncio task and calls exit_actor within it, the actor can still exit correctly.

Related issue number

Closes: #49451

Checks

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  • I've added any new APIs to the API Reference. For example, if I added a
    method in Tune, I've added it in doc/source/tune/api/ under the
    corresponding .rst file.
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• Testing Strategy
  • Unit tests
  • Release tests
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