Hold state store lock while while processing sync response in Client::sync_once

[mgoldenberg]

This pull request ensures that Client::sync_once holds the state store lock while processing a sync response. This amounts to acquiring the lock for the duration of BaseClient::receive_sync_response_with_requested_required_states, which is responsible for processing the response.

Note that this mimics the behavior of SlidingSync::sync_once.

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Closes #6548.

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

Signed-off-by: Michael Goldenberg m@mgoldenberg.net

[codecov]

Codecov Report

❌ Patch coverage is 75.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 89.95%. Comparing base (8675cb2) to head (fb1d04c).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
crates/matrix-sdk-base/src/client.rs	75.00%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6555      +/-   ##
==========================================
- Coverage   89.95%   89.95%   -0.01%     
==========================================
  Files         382      382              
  Lines      107959   107958       -1     
  Branches   107959   107958       -1     
==========================================
- Hits        97118    97109       -9     
- Misses       7172     7176       +4     
- Partials     3669     3673       +4     

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[codspeed-hq]

Merging this PR will not alter performance

✅ 50 untouched benchmarks

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_{Comparing mgoldenberg:hold-state-store-lock-during-sync (fb1d04c) with main (8675cb2)}