feat(sdk): Implement persistent storage for ThreadEventCache

[Hywan]

This patch implement persistent storage for ThreadEventCache.

This contribution deserves a lot more tests but it will increase the diff size by a lot. The basic tests are present though (adding 800 LOC in this patch already!). The current strategy is to ensure all existing tests are passing. A test with a custom build of Element X confirms it works as expected. Another PR will add all the missing tests, along with modifying the test suite of the Event Cache to share many tests between the RoomEventCache and the ThreadEventCache.

This contribution makes it work. However, refactoring is now necessary as similar code exists between RoomEventCache and ThreadEventCache. That's for a next patch.

Best to review one patch at a time. I tried to keep patch as small as possible.

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• Address [meta] Threads #4869
• Address threads: persistent storage #5122

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• I've documented the public API Changes in the appropriate CHANGELOG.md files.
• This PR was made with the help of AI.

[codspeed-hq]

Merging this PR will not alter performance

✅ 50 untouched benchmarks

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_{Comparing Hywan:feat-sdk-event-cache-thread-persistent-storage (f83fbc9) with main (6777907)¹}

Footnotes

1. No successful run was found on event-cache-refactoring (6777907) during the generation of this report, so main (6777907) was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[codecov]

Codecov Report

❌ Patch coverage is 86.62885% with 165 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.85%. Comparing base (dab87b7) to head (f83fbc9).
⚠️ Report is 148 commits behind head on event-cache-refactoring.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
.../matrix-sdk/src/event_cache/caches/thread/state.rs	69.23%	40 Missing and 8 partials ⚠️
...es/matrix-sdk/src/event_cache/caches/thread/mod.rs	95.40%	14 Missing and 22 partials ⚠️
crates/matrix-sdk/src/event_cache/persistence.rs	53.84%	32 Missing and 4 partials ⚠️
...ix-sdk/src/event_cache/caches/thread/pagination.rs	72.80%	25 Missing and 6 partials ⚠️
...es/matrix-sdk/src/event_cache/caches/room/state.rs	85.41%	1 Missing and 6 partials ⚠️
...ix-sdk/src/event_cache/caches/pinned_events/mod.rs	0.00%	3 Missing ⚠️
...ates/matrix-sdk/src/event_cache/caches/room/mod.rs	81.25%	0 Missing and 3 partials ⚠️
...trix-sdk/src/event_cache/caches/room/pagination.rs	90.90%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@                    Coverage Diff                    @@
##           event-cache-refactoring    #6317    +/-   ##
=========================================================
  Coverage                    89.84%   89.85%            
=========================================================
  Files                          378      379     +1     
  Lines                       104337   105292   +955     
  Branches                    104337   105292   +955     
=========================================================
+ Hits                         93746    94608   +862     
- Misses                        7000     7061    +61     
- Partials                      3591     3623    +32     

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[poljar]

I left a couple of nits and questions, besides those this looks good to me.