refactor(timeline): enhance the Timeline::send() and Timeline::send_reply() APIs when the timeline is threaded

[bnjbvr]

This should make it simpler to send in-thread messages and in-thread replies in the future, but not mandating that the caller passes so much information.

• To send a message in a threaded timeline, one can now use Timeline::send()
• To send a reply in a threaded timeline, one can now use Timeline::send_reply() which only takes an OwnedEventId param instead of a Reply
• (To start a new thread, you'd create a threaded timeline and use Timeline::send().)

The improvement is notable in the code of multiverse, for instance, where sending an event is now the same code between a regular or a threaded timeline.

If the threaded timeline isn't empty, it'll correctly use the latest in-thread event as the replied-to fallback event id (a check has been added in one test for that, and this avoids the mess observed in #5422). We could go one step further and trigger a pagination if the threaded timeline was empty; we'll see if we'll ever need it.

cc @Johennes, curious to get your thoughts on the new API!

Fixes #5216

[codecov]

Codecov Report

Attention: Patch coverage is 79.71014% with 14 lines in your changes missing coverage. Please review.

Project coverage is 88.87%. Comparing base (37626b5) to head (f75dca2).
Report is 14 commits behind head on main.

✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
crates/matrix-sdk-ui/src/timeline/futures.rs	71.79%	6 Missing and 5 partials ⚠️
crates/matrix-sdk-ui/src/timeline/mod.rs	88.88%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5427      +/-   ##
==========================================
+ Coverage   88.85%   88.87%   +0.02%     
==========================================
  Files         333      333              
  Lines       91356    91400      +44     
  Branches    91356    91400      +44     
==========================================
+ Hits        81172    81231      +59     
+ Misses       6360     6347      -13     
+ Partials     3824     3822       -2     

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[Johennes]

This sounds sensible and much easier than fiddling with enforce_thread or reply_within_thread. 👍

[bnjbvr]

Stefan identified a few issues, so might as well add some tests later:

• add tests for stickers
• add tests for polls

[stefanceriu]

The new API looks great and I have a PR for EX adopting it here element-hq/element-x-ios#4344

I did leave a comment about polls not working as expected but that's not necessarily a blocker as we didn't have an API for sending them in threads before either.

Same would apply to stickers I guess if we were to ever implement sending support for them.