feat(room): make load_event_with_relations also load relations when falling back to the network

[Johennes]

Room::load_event_with_relations currently tries to load the event and any known relations from the cache first. If that fails, /event is called to retrieve the event itself. Nothing is done to also load the relations, however.

This PR extends load_event_with_relations to also issue /relations requests to load any relations when the cache look-up failed.

• Public API changes documented in changelogs (optional)

[codecov]

Codecov Report

❌ Patch coverage is 85.93750% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.50%. Comparing base (238e4e8) to head (2e6b214).
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...matrix-sdk-ui/src/timeline/pinned_events_loader.rs	72.00%	3 Missing and 4 partials ⚠️
crates/matrix-sdk/src/event_cache/room/mod.rs	93.93%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5930   +/-   ##
=======================================
  Coverage   88.50%   88.50%           
=======================================
  Files         362      362           
  Lines      103291   103351   +60     
  Branches   103291   103351   +60     
=======================================
+ Hits        91413    91471   +58     
+ Misses       7535     7533    -2     
- Partials     4343     4347    +4     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[codspeed-hq]

CodSpeed Performance Report

Merging #5930 will not alter performance

_{Comparing Johennes:johannes/event-with-relations-from-remote (2e6b214) with main (238e4e8)}

Summary

✅ 50 untouched

[andybalaam]

Good catch. Looks good to me but I'm going to ask someone from the Rust team to have a quick check, especially on your question about limiting the number to fetch.

[andybalaam]

A couple of questions to resolve.