threaded timeline: responding to an in-thread message doesn't use the correct fallback

[bnjbvr]

It seems that if one uses a threaded timeline, and answers an in-thread message, the reply fallback's event is set to the thread's root event, not the thread's latest event. It's unclear whether it's a bug in EXI or in the SDK, I'll investigate when I'm back.

[bnjbvr]

When sending an in-thread message, that's not an explicit reply to another in-thread message, EXI will say it's a response to the thread root:

https://github.com/element-hq/element-x-ios/blob/57469fb8eb1476480c8c82b28de0ae5356abb005/ElementX/Sources/Services/Timeline/TimelineProxy.swift#L468-L472

It's incorrect: the replied-to event id should always be set to the latest in-thread message, for the fallback to work as intended.

Arguably, the API is bad here, so closing as a dup of #5216.

I think another action to take would be for EXI to fix the API usage as well, until #5216 is fixed, cc @stefanceriu 🥺