Use explicit sort key in ConcurrentExecutorListResults

[dkropachev]

Summary

Fixes bug in response handling:

TypeError: '<' not supported between instances of 'dict' and 'list'

• Sort results by index only via key=lambda x: x[0], avoiding unnecessary comparison of ExecutionResult namedtuples which could fail on heterogeneous result/exception types

Fixes #715

Test plan

• Existing execute_concurrent tests cover correctness of result ordering
• Single-line defensive change — no behavioral difference for normal operation

[Lorak-mmk]

I first sent an approve, and only then read an issue. The "problem" this fixes only happens if ids are equal. IDs can never be equal, because of how statements are generated: enumerate(iter(statements_and_params)).

This is not just some immaterial detail that could change in the future: user needs to get results in the same order they passed the requests, in order to be able to match them. This means this "bug" can never be triggered. So what problem is solved here?

[dkropachev]

I first sent an approve, and only then read an issue. The "problem" this fixes only happens if ids are equal. IDs can never be equal, because of how statements are generated: enumerate(iter(statements_and_params)).

This is not just some immaterial detail that could change in the future: user needs to get results in the same order they passed the requests, in order to be able to match them. This means this "bug" can never be triggered. So what problem is solved here?

Yuu are right, technically it is unlikely, but in some cases it happens, here is the example:
https://github.com/scylladb/python-driver/actions/runs/22284500171/job/64460379139

[Lorak-mmk]

If it happens, it indicates a more serious bug. Instead of silencing it with the fix in this PR, it needs to be fixed.
Sure, the fix here would prevent the specific exception, but it would make return value meaningless (because results would have no well-defined sort order).