fix(core): Identify custom `AggregateError`s as exception groups

[github-actions]

Note

The pull request "fix(core): Identify custom AggregateErrors as exception groups" was created by @Lms24 but did not reference an issue. Therefore this issue was created for better visibility in external tools like Linear.

Before submitting a pull request, please take a look at our
Contributing guidelines and verify:

• If you've added code that should be tested, please add tests.
• Ensure your code lints and the test suite passes (yarn lint) & (yarn test).
• Link an issue if there is one related to your pull request. If no issue is linked, one will be auto-generated and linked.

Closes #issue_link_here

[linear]

JS-1584