Add wait_until_ready=True to sandbox snapshot task id retrieval

[azliu0]

Describe your changes

I was playing around with trying to break snapshots (see here) and realized I forgot to include this earlier.

For context, this flag will more gracefully propagate snapshot failures to the client. Without this flag, it may be possible to exec something on the broken snapshot and receive a container exited failure instead.

Backward/forward compatibility checks

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Check these boxes or delete any item (or this section) if not relevant for this PR.

• Client+Server: this change is compatible with old servers
• Client forward compatibility: this change ensures client can accept data intended for later versions of itself

Note on protobuf: protobuf message changes in one place may have impact to
multiple entities (client, server, worker, database). See points above.

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[azliu0]

@prbot approve

[modal-pr-review-automation]

Approved 👍. @thecodingwizard will follow-up review this.

[thecodingwizard]

lgtm!

[thecodingwizard]

Oops actually @azliu0 I think we need to explicitly specify the timeout field here. The server code uses the presence of the timeout field to determine whether it is a legacy client that expects task_id="" or a new client that expects a DEADLINE_EXCEEDED error.