fix: Refresh timeline items when their sender's avatar URL changes

[jmartinesp]

This is similar to what happens when a user's display name changes, its ambiguity is calculated and if it changes, it reloads the associated timeline events.

In fact, this change tries to follow the same strategy as AmbiguityCache.

Fixes #6575.

• 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:

[codecov]

Codecov Report

❌ Patch coverage is 94.39252% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.95%. Comparing base (2a8d75e) to head (97ccd3b).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
crates/matrix-sdk-base/src/store/avatar_cache.rs	94.91%	3 Missing ⚠️
crates/matrix-sdk-ui/src/timeline/tasks.rs	81.81%	2 Missing ⚠️
...x-sdk-base/src/response_processors/state_events.rs	66.66%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6576   +/-   ##
=======================================
  Coverage   89.94%   89.95%           
=======================================
  Files         381      382    +1     
  Lines      107718   107812   +94     
  Branches   107718   107812   +94     
=======================================
+ Hits        96889    96980   +91     
- Misses       7160     7161    +1     
- Partials     3669     3671    +2     

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[codspeed-hq]

Merging this PR will not alter performance

✅ 50 untouched benchmarks

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_{Comparing fix/refresh-timeline-items-when-sender-changes-avatar-url (97ccd3b) with main (2a8d75e)}

[poljar]

Thanks.

The tests were quite useful to confirm this.