A `message.part.updated` snapshot did not invalidate the pending delta
coalescing key for its message/part. A delta arriving after an intervening
snapshot merged into a delta queued before it, and the snapshot then
overwrote that slot, dropping the later delta's text (e.g. `abc` rendered
as `ab`). Enqueueing a part snapshot now drops that part's pending delta
coalescing keys, while leaving already-queued delta events in place, so
post-snapshot deltas start a fresh entry.
Closes#1647.
Co-authored-by: Ibrahim Khan <ibrakhxn@amazon.com>
