fix: Add missing animations to discreet split panel (2)

[jperals]

Description

Recreation of #3322 with a fix for the closed discreet split panel displaying a double shadow in Classic.

Ticket: AWSUI-60450

How has this been tested?

In my pipeline

Review checklist

The following items are to be evaluated by the author(s) and the reviewer(s).

Correctness

• Changes include appropriate documentation updates.
• Changes are backward-compatible if not indicated, see CONTRIBUTING.md.
• Changes do not include unsupported browser features, see CONTRIBUTING.md.
• Changes were manually tested for accessibility, see accessibility guidelines.

Security

• If the code handles URLs: all URLs are validated through the checkSafeUrl function.

Testing

• Changes are covered with new/existing unit tests?
• Changes are covered with new/existing integration tests?

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.42%. Comparing base (c4a43a9) to head (b9c3116).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3339   +/-   ##
=======================================
  Coverage   96.42%   96.42%           
=======================================
  Files         791      791           
  Lines       22611    22616    +5     
  Branches     7814     7811    -3     
=======================================
+ Hits        21803    21808    +5     
- Misses        754      801   +47     
+ Partials       54        7   -47     

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