Follow on from https://github.com/element-hq/synapse/pull/19701.
Unfortunately serde has a bug when using `#[serde(flatten)]` with
`arbitrary-precision` feature when handling integers that fit in a i128
when doing `serde_json::from_value`. See
https://github.com/serde-rs/serde/issues/2230.
The `depythonize` hits the same issue. To fix this we make it so we only
parse events from strings and not values.