fix: Position drag UAPs correctly on same-origin iframes sharing a React tree

[avinashbot]

Description

Crazy what you can do with an iframe these days. Rather than trying to do anything complex with calculating parent iframes and offsets, I just decided to put the portal in the closest iframe so all the rest of the assumptions work out.

Related links, issue #, if available: AWSUI-60742

How has this been tested?

Screenshot tests are probably the best way to handle this. Since the feature has already made it through, I'll add the screenshot tests first, and then merge this.

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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 96.56%. Comparing base (6a94647) to head (b90ee8b). Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #3502    +/-   ##
========================================
  Coverage   96.55%   96.56%            
========================================
  Files         807      807            
  Lines       23456    23461     +5     
  Branches     8182     7758   -424     
========================================
+ Hits        22648    22654     +6     
- Misses        754      800    +46     
+ Partials       54        7    -47     

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