Improve ExpectNextNAsync error message clarity

[CodingPythonMan]

Previous error messages were unclear when ExpectNextNAsync failed, especially in tests (e.g., with Kafka). The new message includes the exact index of the mismatch and shows both expected and actual values.

Old:
"Expected one of (1, 2, 3, ...), but got '5'"

New:
"Expected element at index 24 to be '25', but got '5'"

This improves debugging clarity for stream-based tests and addresses developer confusion in cases with large input sequences.

Fixes #7536

Changes

Updated ExpectNextNTask to provide clearer error messages with exact mismatch index and values.

Checklist

For significant changes, please ensure that the following have been completed (delete if not relevant):

• This change follows the Akka.NET API Compatibility Guidelines.
• This change follows the Akka.NET Wire Compatibility Guidelines.
• I have reviewed my own pull request.
• Design discussion issue Akka.Streams.TestKit: ExpectNextNAsync unfriendly error message #7536

[Aaronontheweb]

LGTM

[Aaronontheweb]

You had a racy spec failure here and it's one of the tests we're working on fixing in #7615 and #7579, so not your code's fault

[Aaronontheweb]

@CodingPythonMan congrats on your first contribution to Akka.NET 🥳

[CodingPythonMan]

Thank you so much, @Aaronontheweb 🙏
I’m really glad to hear that the failure wasn’t caused by my change, and I appreciate your kind words.
This was a fun one to work on — I learned quite a bit, and I’m excited to keep exploring and contributing where I can!