refactor(timeline): add some logs when an embedded event isn't a standalone item

[bnjbvr]

This should help in two cases:

• when the latest event (using the new API) is creating an aggregation, for some reason,
• when the latest thread reply is an aggregation, for some reason.

At this, this should help with figuring out why a thread's latest reply is sometimes "unsupported", as reported in #5702

[codecov]

Codecov Report

❌ Patch coverage is 31.57895% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 88.39%. Comparing base (59b7da2) to head (b52fd19).
⚠️ Report is 3 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
crates/matrix-sdk-ui/src/timeline/event_handler.rs	23.07%	10 Missing ⚠️
crates/matrix-sdk-ui/src/timeline/latest_event.rs	50.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5730      +/-   ##
==========================================
- Coverage   88.40%   88.39%   -0.02%     
==========================================
  Files         359      359              
  Lines       99048    99051       +3     
  Branches    99048    99051       +3     
==========================================
- Hits        87566    87555      -11     
- Misses       7344     7362      +18     
+ Partials     4138     4134       -4     

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[codspeed-hq]

CodSpeed Performance Report

Merging #5730 will not alter performance

_{Comparing bnjbvr/log-embedded-events (b52fd19) with main (59b7da2)}

Summary

✅ 50 untouched

[stefanceriu]

lgtm 👍