repeat_* sender completion signature oddities.

[justend29]

Two questions about some unexpected behaviours of the repeat_ family of senders in the exec namespace:

1. repeat_effect completely omits completion signatures.

   Although I presume this was done because the sender never completes on the value channel, it's counterintuitive and extremely limiting to omit the completion signatures. Where repeat_effect means "repeat upstream sender until error or exception", I immediately presumed the completion signatures would simply never contain set_value_t(). Without advertising completion signatures, any sender using repeat_effect essentially cannot be composed with any further senders. Examples that won't compile:

      auto snd_1 = stdexec::just_stopped()
         | exec::repeat_effect()
         | stdexec::upon_stopped([](){return -1;}); // cannot chain w/o completion_signatures

     // starts_on tries to transform completion signatures that don't exist
     exec::timed_thread_context ctx{};
     auto snd_2 = stdexec::starts_on(ctx.get_scheduler(), stdexec::just_stopped() | exec::repeat_effect());

   Is this intentional or a bug? If it's intentional, how is repeat_effect supposed to be used?

2. repeat_effect_until and repeat_n add stdexec::set_stopped_t() to the completion signatures.

   Where the upstream senders in the following two examples complete only on the value channel, why would the repeat adapters transform the completion signatures as such? I gather that the stdexec::set_error_t(std::exception_ptr) signature is added to account for possible exceptions within the repeat_* machinery, and querying all the exception guarantees of creating the op-state to conditionally add an error channel would be difficult. Is the stop channel similarly added as a blanket solution?

     auto snd = stdexec::just(false) | exec::repeat_effect_until();
     static_assert(std::same_as<
       stdexec::completion_signatures_of_t<decltype(snd)>, 
       stdexec::completion_signatures<
         stdexec::set_stopped_t(),
         stdexec::set_error_t(std::exception_ptr),
         stdexec::set_value_t()
       >
     >);
   
     auto snd_n = stdexec::just() | exec::repeat_n(3);
     static_assert(std::same_as<
       stdexec::completion_signatures_of_t<decltype(snd_n)>, 
       stdexec::completion_signatures<
         stdexec::set_value_t(),
         stdexec::set_stopped_t(),
         stdexec::set_error_t(std::exception_ptr)
       >
     >);

Thanks for any feedback!

Examples were tested with the current tip of main, 8cfc3f1