Fix duplicated exception chain output for exceptions without a traceback

[SemTiOne]

When an exception in a chain has no __traceback__ of its own, repr_excinfo() falls back to traceback.format_exception(), which defaults to chain=True and walks the rest of the chain itself, duplicating what pytest's own chain-walking loop then prints again right after.

Fix: pass chain=False to that fallback call.

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• Closes Exceptions without a traceback print multiple times if chained #8321
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[SemTiOne]

Hello @RonnyPfannschmidt, I would like you to take a look at this PR when you have a moment please. Let me know if you have any feedback. Thank you for your time 🙏

[RonnyPfannschmidt]

am i understanding it correct that this bandaids our lack of proper walking with disabling chaining to prevent running out of it

i think this works as a initial fix but needs a followup with more substantial work in excinfo wrt handling more detailed traces

[SemTiOne]

Yep, I agree this is a bandaid.

I wonder if we should make the traceback optional in ExceptionInfo? Or, perhaps we add a lightweight chain member representation? With repr_excinfo as the one and only chain walker, no native format_exception fallback is needed for traceback-less members. The ExceptionGroup fallback stays native for now and we no longer need the chain=False workaround. What do you think @RonnyPfannschmidt?