refactor(timeline): allow creating a TimelineItemContent from a sdk::TimelineEvent

[bnjbvr]

A small refactor extracted from the search PR, as it's actually useful to do ahead of time, since the code was already duplicated.

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

[Hywan]

LGTM, thanks.

[codecov]

Codecov Report

❌ Patch coverage is 84.31373% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.86%. Comparing base (32e9626) to head (2fe657b).
⚠️ Report is 1 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...rates/matrix-sdk-ui/src/timeline/event_item/mod.rs	69.23%	3 Missing and 1 partial ⚠️
...trix-sdk-ui/src/timeline/event_item/content/mod.rs	88.46%	2 Missing and 1 partial ⚠️
crates/matrix-sdk-ui/src/timeline/latest_event.rs	87.50%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6408      +/-   ##
==========================================
+ Coverage   89.85%   89.86%   +0.01%     
==========================================
  Files         378      378              
  Lines      103384   103345      -39     
  Branches   103384   103345      -39     
==========================================
- Hits        92893    92873      -20     
+ Misses       6928     6905      -23     
- Partials     3563     3567       +4     

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[codspeed-hq]

Merging this PR will not alter performance

✅ 50 untouched benchmarks

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_{Comparing bnjbvr/timeline-item-content (2fe657b) with main (32e9626)}