Fix NoneType error in get_conversation and other typesafety

[raymyers]

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End-user friendly description of the problem this fixes or functionality this introduces.

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Summarize what the PR does, explaining any non-trivial design decisions.

Adding the type annotation to conversation_manager in shared.py makes it so mypy catches the missing None check behind this recent runtime exception:

openhands/server/conversation_manager/standalone_conversation_manager.py", line 136, in detach_from_conversation
    sid = conversation.sid
          ^^^^^^^^^^^^^^^^
AttributeError: 'NoneType' object has no attribute 'sid'

It also appears to have flagged a missing async. It's possible that these constants have left a larger hole in our type-checking. I originally tried to solve this by forcing return type annotations in #8911 (maybe still a good idea), but this turned out to me enough.

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Link of any specific issues this addresses:

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docker run -it --rm   -p 3000:3000   -v /var/run/docker.sock:/var/run/docker.sock   --add-host host.docker.internal:host-gateway   -e SANDBOX_RUNTIME_CONTAINER_IMAGE=docker.all-hands.dev/all-hands-ai/runtime:d2c5c00-nikolaik   --name openhands-app-d2c5c00   docker.all-hands.dev/all-hands-ai/openhands:d2c5c00

[neubig]

This will silently do nothing if agent_loop_info.event_store is None, is that the proper behavior? Or is the proper behavior to throw an error?

[raymyers]

That's a good question, I don't know enough about the scenario it could be None in. My thinking was "Well we're looking over all the events so if there's no store there are definitely no events to loop over".

But I can equally argue that AsyncEventStoreWrapper would have exploded before so we should preserve the behavior unless we know we want different.

[raymyers]

@tofarr @malhotra5 Would you know more about the desired behavior here?

[raymyers]

This is the one from the prod error.

[raymyers]

Probably this was a hidden performance bug.

[openhands-ai]

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[raymyers]

Partially done in #8927, going to do the rest later.