conditionally contribute set_error_t(std::exception_ptr) in repeat_* senders

[justend29]

Currently, the repeat_ family of sender adapters invariantly contributes set_error_t(std::exception_ptr) to its completion signatures, which I first mentioned in this issue. This PR causes repeat_until, repeat, and repeat_n to conditionally contribute set_error_t(std::exception_ptr) to the completion signatures. Each of the possible cases (not) to contribute the completion signature are individually tested.

It also contains some small corrections that were originally in #1797, until @RobertLeahy informed me that it is possible to detect the noexcept-ness of a connect using STDEXEC::__receiver_archetype<Env>, wherein I returned to this original goal.

This is also my first time working with the meta-programming system in this library, so feedback is welcome.

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[justend29]

Thank you for the review, @ericniebler. All of your comments have been addressed, though __partition_completion_signatures_t was changed to __partitions_of_t to compile.

[ericniebler]

/ok to test ac8352c

[ericniebler]

/ok to test 99cf087

[justend29]

@ericniebler It's odd that the CI jobs using an older version of GCC fail while the other CI jobs pass. I tested everything with GCC15.2.1 locally. I'm not sure if you have any ideas, but I can probably install an older version of GCC and try to reproduce the failing build locally over the weekend.

[ericniebler]

It's odd that the CI jobs using an older version of GCC fail while the other CI jobs pass.

that's business as usual. getting CI to pass with older, less-conforming compilers is often the majority of the work for PRs. i wish things were different.