Fix thread edit aggregation race condition (#4980)
* test(thread): add regression tests for edit-race; ensure reaction aggregation idempotence
- Edit race: add failing test when `Replace` aggregated pre-init
- Reaction: ensure pre-init aggregation and dedup on replay
- Strengthen assertions for ordering and content
* fix(thread): apply edits after init; keep reactions pre-init; remove redundant aggregation
- Defer Replace aggregation until thread initialised and event is in timeline
- Aggregate Annotation pre-init to preserve reaction summaries
- Rely on EventTimelineSet to aggregate post-insert
- Fixes: element-hq/element-web#30617
* style: run prettier; docs: clarify reaction pre-init comment about counts
