feat(sdk): All caches have an auto-shrink mechanism

[Hywan]

During the big refactoring of the Event Cache, I noticed that the auto-shrink mechanism needs to be implement on all caches now. It was implemented on RoomEventCache before, but we want to extend it to ThreadEventCache now its supports persistent storage (see #6317), and maybe on PinnedEventsCache too since it has its own storage too (note that this is less critical as it seems unlikely we will have that many pinned-events living in memory).

Tasks

• Move event_cache/caches/room/subscriber.rs to event_cache/caches/subscriber.rs
• Transform RoomEventCacheSubscriber to Subscriber<RoomEventCacheUpdate>, i.e. make it generic over any updates,
• Add tests for this Subscriber type only,
• Update AutoSkrinkPayload to be an enum instead of OwnedRoomId, something like:

  enum AutoShrinkPayload {
      Room { room_id: OwnedRoomId },
      Thread { room_id: OwnedRoomId, thread_id: OwnedEventId },
       // etc.
  }

• Update the auto_shrink_linked_chunk_task,
• Update ThreadEventCache::subscribe to return a Subscriber<TimelineVectorDiffs instead of a Receiver<TimelinevectorDiffs>,
• Update PinnedEventsCache::subscribe to return a Subscriber<TimelineVectorDiffs instead of a Receiver<TimelinevectorDiffs>.

---

• Address [meta] Event Cache layers refactoring #6152

---

• I've documented the public API changes in the appropriate changelog files (see Writing changelog entries).
• This PR was made with the help of AI.

Signed-off-by:

[codecov]

Codecov Report

❌ Patch coverage is 96.06557% with 12 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.91%. Comparing base (567463d) to head (ffb8479).
⚠️ Report is 5 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...es/matrix-sdk/src/event_cache/caches/thread/mod.rs	97.36%	0 Missing and 3 partials ⚠️
crates/matrix-sdk/src/event_cache/tasks.rs	72.72%	3 Missing ⚠️
...rates/matrix-sdk-ui/src/timeline/controller/mod.rs	60.00%	0 Missing and 2 partials ⚠️
...es/matrix-sdk/src/event_cache/caches/subscriber.rs	98.65%	2 Missing ⚠️
.../matrix-sdk/src/event_cache/caches/thread/state.rs	85.71%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6690      +/-   ##
==========================================
- Coverage   89.91%   89.91%   -0.01%     
==========================================
  Files         396      396              
  Lines      110475   110688     +213     
  Branches   110475   110688     +213     
==========================================
+ Hits        99338    99523     +185     
- Misses       7371     7382      +11     
- Partials     3766     3783      +17     

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[codspeed-hq]

Merging this PR will not alter performance

✅ 50 untouched benchmarks

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_{Comparing Hywan:feat-sdk-event-cache-generalised-auto-shrinker (ffb8479) with main (5a0b002)}