actions_base: reimplement HPX_DEFINE_PLAIN_ACTION_2 using C++26 reflection

[Priyanshi507]

Summary

When HPX_HAVE_CXX26_REFLECTION is defined, HPX_DEFINE_PLAIN_ACTION_2
now uses reflect_action<^^func> internally:

Before:

struct name : hpx::actions::make_action_t<decltype(&func), &func, name> {};

After (with reflection):

using name = hpx::actions::reflect_action<^^func>;

Key insight

The first macro argument func is a named entity at the call site —
not a template parameter — so ^^func works correctly on both
GCC trunk and Clang P2996.

User impact

Zero — same macro syntax, reflection used internally:

HPX_PLAIN_ACTION(app::compute, compute_action)  // unchanged

Backwards compatibility

Non-reflection paths (NVCC/CUDACC and standard C++) are preserved unchanged.

Checklist

• I have added a new feature and have added tests to go along with it.

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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[StellarBot]

Can one of the admins verify this patch?

[Priyanshi507]

@hkaiser
Converting this to draft the macro change is correct but \reflect_actionneeds to inherit frombasic_action<detail::plain_function, func_type, reflect_action> to provide the required typedefs (component_type, result_type, etc.) for transfer_action. I've verified the two-step approach works in isolation. The next PR will add this inheritance to reflect_action and then this macro change will follow. Marking as draft until then.