fix(exec): allow any_sender completion signatures through reference types

[alwaysprince05]

Summary

Fixes #2101 (supersedes #2223 with a simpler, correct approach).

When exec::sequence has 3+ senders, the recursive completion-signature
computation in __seq::__sndr produces reference-qualified sender types
(e.g. const any_sender&). The constraint derived_from<Self, _interface_>
on _isender::_interface_::get_completion_signatures() rejects these because
std::derived_from is never satisfied by a reference type.

Fix

Decay Self before the derived_from check. This is a 2-line change in
any_sender_of.hpp:

-        template <__std::derived_from<_interface_> Self, class... Env>
+        template <class Self, class... Env>
+          requires __std::derived_from<__decay_t<Self>, _interface_>
         static consteval auto get_completion_signatures()

get_completion_signatures is a static consteval function that returns
type information only — it never connects the sender, so the cvref-qualifiers
on Self are irrelevant. The connect member function remains &&-qualified
and still rejects const lvalue references.

Why this approach

The earlier PR #2223 changed __attrs to decay children types and
__sequence.hpp to decay the first sender for signature queries. Eric noted
that "the cvref-ness of the first sender matters" for connect (line 328).
Our fix avoids touching __sequence.hpp entirely — the constraint was simply
too restrictive for a function that only computes types.

Verified

• 2 any_senders ✅
• 3 any_senders ✅ (previously failed to compile)
• 4 any_senders ✅
• Concrete senders only ✅ (regression check)
• Mixed concrete + any_senders ✅

🤖 Generated with Codebuff
Co-Authored-By: Codebuff noreply@codebuff.com

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[ericniebler]

i'm afraid this fix is wrong too. if sender_in<S, E> is true, then a sender of type S should be connect-able with a receiver whose environment has type E. after this change, that is no longer true for any_sender. only rvalue any_senders can be connected, so any_sender::get_completion_signatures is right to reject lvalue any_senders.