Add enter animation to new timeline events

[jmartinesp]

Content

Add the WithEnterAnimation wrapper to add an slide-up + fade in animation to timeline events with SYNC or LOCAL origin so new items either coming from sync or sent by the current users animate when added to the timeline. Other items will work the same.

Motivation and context

Implement #6966.

FWIW, I tried a more thorough implementation with Modifier.animateItem() but there were lots of weird animations, as we had before:

• Grouped events overlapping with any events above them when being expanded.
• When you send several items in a row, the latest event with the 'sent' indicator will collapse with an animation, and the new item being added will be animating from what it looks like the middle of that event, which makes them overlap in an awkward way.
• Probably more I haven't noticed.

So I decided to drop that implementation and do something simpler.

Screenshots / GIFs

Screen_recording_20260910_145639.mp4

Tests

Test sending and receiving new events in the timeline, as well as redacting them or paginating.

Tested devices

• Physical
• Emulator
• OS version(s): 15

Checklist

• I am aware of the etiquette.
• This PR was made with the help of AI:
  • Yes. In this case, please request a review by Copilot.
  • No.
• Changes have been tested on an Android device or Android emulator with API 24
• UI change has been tested on both light and dark themes
• Accessibility has been taken into account. See https://github.com/element-hq/element-x-android/blob/develop/CONTRIBUTING.md#accessibility
• Pull request is based on the develop branch
• Pull request title will be used in the release note, it clearly defines what will change for the user
• Pull request includes screenshots or videos if containing UI changes
• You've made a self review of your PR

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[sonarqubecloud]

Quality Gate failed

Failed conditions
20.4% Duplication on New Code (required ≤ 10%)

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

❌ Patch coverage is 72.34043% with 13 lines in your changes missing coverage. Please review.
✅ Project coverage is 80.90%. Comparing base (c623105) to head (4e47b77).
⚠️ Report is 2 commits behind head on develop.

Files with missing lines	Patch %	Lines
...ssages/impl/timeline/components/TimelineItemRow.kt	72.34%	9 Missing and 4 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #7686      +/-   ##
===========================================
- Coverage    80.91%   80.90%   -0.02%     
===========================================
  Files         2780     2780              
  Lines        81899    81944      +45     
  Branches     11284    11289       +5     
===========================================
+ Hits         66265    66293      +28     
- Misses       11290    11299       +9     
- Partials      4344     4352       +8     

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[bxdxnn]

Funny, I've also started working on it recently and was able to get a smooth animation for the entire timeline instead of just a fade-in for the new message. It required some refactoring instead of just putting the row in a box, could you please take a look?
#7688