fix: presence getting stuck

[FrenchGithubUser]

this has been largely written by claude. it claims to have found the issue, fixed it and tested the fix, but I haven't verified it all. double check things!

[codecov]

Codecov Report

❌ Patch coverage is 90.90909% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 80.73%. Comparing base (013821e) to head (2523ecb).

Files with missing lines	Patch %	Lines
synapse/storage/util/id_generators.py	90.90%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #280      +/-   ##
==========================================
- Coverage   80.73%   80.73%   -0.01%     
==========================================
  Files         503      503              
  Lines       72286    72296      +10     
  Branches    10872    10873       +1     
==========================================
+ Hits        58363    58371       +8     
- Misses      10658    10659       +1     
- Partials     3265     3266       +1     

Files with missing lines	Coverage Δ
synapse/storage/util/id_generators.py	90.72% <90.90%> (-0.03%)	⬇️

... and 1 file with indirect coverage changes

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[FrenchGithubUser]

closing in favor of the upstream PR: element-hq/synapse#20090