feat(threads): convert the types exposed from Room.list_threads to UI crates TimelineItem types

[stefanceriu]

Following #5953 this PR first correctly exposes ThreadRoots fields over FFI and then modifies the whole flow so UI crate timeline types are used instead, briging threads closer to what we do for latest events, reply details etc.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 65 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.85%. Comparing base (af8236d) to head (f379c4f).
⚠️ Report is 35 commits behind head on main.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
crates/matrix-sdk-ui/src/timeline/threads.rs	0.00%	62 Missing ⚠️
crates/matrix-sdk-ui/src/timeline/traits.rs	0.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6278      +/-   ##
==========================================
- Coverage   89.90%   89.85%   -0.06%     
==========================================
  Files         372      373       +1     
  Lines      102463   102528      +65     
  Branches   102463   102528      +65     
==========================================
+ Hits        92123    92126       +3     
- Misses       6787     6850      +63     
+ Partials     3553     3552       -1     

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[codspeed-hq]

Merging this PR will improve performance by 69.42%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 1 improved benchmark
✅ 49 untouched benchmarks

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	Restore session [memory store]	282 ms	166.4 ms	+69.42%

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_{Comparing stefan/threadLists (f379c4f) with main (acda2e8)}