fix(sdk): Fix sticky local latest event

[Hywan]

Based on top of #5947. Must be merged before this one.

The problem we are trying to solve is the following:

• a local event is being sent,
• the LatestEventValue is LocalIsSending,
• the local event is finally sent,
• the LatestEventValue is still LocalIsSending purposely, with the hope that an update from the Event Cache will replace it, thus becoming a Remote.

But sometimes, this update from the Event Cache comes before the update from the Send Queue because the whole mechanism is async with no ordering guarantee. Why is it problem? Because updates from the Event Cache are ignored until the buffer of local LatestEventValues aren't empty (so that local latest event cannot be overwritten and has the priority). It means that if the update from the Event Cache is received before RoomSendQueueUpdate::SentEvent, it is ignored, and the LatestEventValue stays in the LocalIsSending state. That's annoying.

The idea is to introduce a new state: LocalHasBeenSent which mimics Remote, but for a local event. It clarifies the state of a sent event, without relying on the Event Cache.

The patch is twofold:

1. It introduces the new LatestEventValue::LocalHasBeenSent variant,
2. It updates the latest_event Room List sorter to handle this new case.

---

• Address [meta] Latest Event API, the missing one #4112

[codecov]

Codecov Report

❌ Patch coverage is 92.34450% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.51%. Comparing base (1480ede) to head (db6684a).
⚠️ Report is 3 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
crates/matrix-sdk-ui/src/timeline/latest_event.rs	78.57%	1 Missing and 8 partials ⚠️
...rates/matrix-sdk/src/latest_events/latest_event.rs	92.10%	6 Missing ⚠️
...k-ui/src/room_list_service/sorters/latest_event.rs	98.55%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #5968    +/-   ##
========================================
  Coverage   88.51%   88.51%            
========================================
  Files         363      363            
  Lines      103490   103590   +100     
  Branches   103490   103590   +100     
========================================
+ Hits        91599    91696    +97     
  Misses       7532     7532            
- Partials     4359     4362     +3     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[codspeed-hq]

CodSpeed Performance Report

Merging #5968 will not alter performance

_{Comparing Hywan:fix-sdk-latest-event-sticky-locals (db6684a) with main (1480ede)}

Summary

✅ 50 untouched

[poljar]

Makes sense.