Do not allow sending events responses while handling RPC messages

[dfalbel]

Fixes an issue while running viewing a small polars data frame. In this case, the background tasks that return the null cournts would be processed faster than the actual RPC message, leading to an issue where the send_event is sent before send_result, but with the message id from send_result, which cause positron to interpret the event and the return value of the RPC message.

This PR makes sure send_event waits for for the RPC message to be handled before sending the event results.

import polars as pl

from datetime import date
df = pl.DataFrame(
    {
        "foo": [1, 2, 3],
        "bar": [6.0, 7.0, 8.0],
        "ham": [date(2020, 1, 2), date(2021, 3, 4), date(2022, 5, 6)],
        "a": [None, 2, 3],
        "b": [0.5, None, 2.5],
        "c": [True, None, False],
    }
)


%view df

[wesm]

This seems correct to me — the behavior can only be triggered when events may be fired from a background thread (i.e. with #4326) while a request from the main request handling loop is in progress