fix: Proper async handling for portals

[just-boris]

Notes

Fix for this issue: AWSUI-61285.

Technically this API change is breaking. I made sure no consumers will be affected by this change. Dry-run id: 7586509761

Testing

1. Manually tested the original issue using the patched code, it works now
2. The PR build is expected to fail, because there is an API snapshot diff. I will follow up with an update (chore: Follow up for the modal root fixes components#4016, currently in drafts)

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.78%. Comparing base (650399a) to head (bf8e451).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #172   +/-   ##
=======================================
  Coverage   98.77%   98.78%           
=======================================
  Files          36       36           
  Lines         900      905    +5     
  Branches      241      242    +1     
=======================================
+ Hits          889      894    +5     
  Misses         11       11           

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