Move the synchronous core of client event serialization out of
`synapse/events/utils.py` and into `rust/src/events/serialize.rs`. The
Python `EventClientSerializer` now performs all DB/IO (fetching
redactions, running module callbacks, resolving admin/MSC4354 config) up
front in `_prepare_serialization`, then hands the pre-fetched data to the
Rust `serialize_event`, which recurses entirely in Rust. Bundled
aggregations are read directly from the Rust `BundledAggregations`
pyclass.
`SerializeEventConfig` and the `event_format` callable become a Rust
pyclass and the `EventFormat` enum respectively; call sites are updated
to pass the enum.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
