[sequence_senders] item_types_of_t probes sequence item senders with the regular __debug_sender when STDEXEC_ENABLE_EXTRA_TYPE_CHECKING=ON

Requested by @ericniebler in #2271 — follow-up found while verifying that PR with -DSTDEXEC_ENABLE_EXTRA_TYPE_CHECKING=ON. Fails on pristine main, so it is separate from the fix in #2271.

Description

With extra type checking enabled, item_types_of_t (and its __item_types_of helpers) still route sequence item senders through the regular debug sender machinery (__debug_sender), which calls connect() on them. Sequence senders are not regular senders, so compilation fails with:

no matching function for call to object of type 'stdexec::connect_t'

#2271 fixed the analogous paths (__sequence_receiver_from, subscribe_t::__type_check_arguments) by switching to __sequence_completion_signatures_of_t; the item-type probing paths need the same sequence-aware treatment (or a sequence-aware equivalent of __debug_sender).

Reproduction

Compose an exec::any_sequence_sender pipeline and build with the flag enabled. After #2271, the remaining sequence-test errors under the flag largely come from this path (error counts across the sequence test files drop from 20/58/66 on main to 4/6/16 with #2271).

Start by tracing item_types_of_t and the __item_types_of helpers, then compare their regular __debug_sender path with the sequence-aware paths named in the issue. Reproduce an exec::any_sequence_sender pipeline with STDEXEC_ENABLE_EXTRA_TYPE_CHECKING=ON and run the sequence test files; done means those probes compile without the connect() error.