fix(unread counts): a gappy sync shouldn't decrease the unread counts

[bnjbvr]

See code comments and commits descriptions :) This fixes a situation where a gappy sync could cause unread counts to decrease (because unread counts computation rely on the linked-chunk to be completely loaded up to the latest active read receipt).

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

Part of #4113

[codecov]

Codecov Report

❌ Patch coverage is 45.45455% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.90%. Comparing base (b6c4bca) to head (030af23).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...es/matrix-sdk/src/event_cache/caches/room/state.rs	45.45%	2 Missing and 4 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6284   +/-   ##
=======================================
  Coverage   89.90%   89.90%           
=======================================
  Files         374      374           
  Lines      102613   102624   +11     
  Branches   102613   102624   +11     
=======================================
+ Hits        92250    92266   +16     
+ Misses       6798     6789    -9     
- Partials     3565     3569    +4     

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[codspeed-hq]

Merging this PR will degrade performance by 60.29%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

❌ 1 regressed benchmark
✅ 49 untouched benchmarks

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
❌	Simulation	Restore session [memory store]	111.9 ms	281.8 ms	-60.29%

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_{Comparing bnjbvr/event-cache-shrink-unread-counts (030af23) with main (b6c4bca)}