drop dead trait, dead test guards, and migrate async_mpi receiver to native P2300

[guptapratykshh]

Proposed Changes

• Removed dead HPX_HAVE_STDEXEC test guards left over from Clean up stdexec-only paths and update affected tests #7123 — deleted test_tag_invoke_only_completion_signatures.cpp (its body was wrapped in #if !defined(HPX_HAVE_STDEXEC) so it never executed) and the 5 always-false guarded blocks in coroutine_utils.cpp that tested HPX's removed awaitable-as-sender path.
• Deleted the dead has_completion_signatures trait from completion_signatures.hpp. It checked for the nested ::completion_signatures alias that the removed HPX fallback used, with stdexec mandatory, completion signatures come exclusively via tag_invoke(get_completion_signatures_t, ...) or stdexec's standard discovery. Also removed its one remaining live use in the completion_signatures test.
• Migrated async_mpi's transform_mpi_receiver and transform_mpi_sender from friend tag_invoke(set_X_t/connect_t, ...) to the modern member-based form (void set_value(...) && noexcept, auto connect(R&&) &[/&&], using receiver_concept = receiver_t). This brings them in line with the shape that keep_future and async_cuda's transform_stream already use. The tag_invoke(get_completion_signatures_t, ...) was kept as that customization point has no member-function form.

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Checklist

Not all points below apply to all pull requests.

• I have added a new feature and have added tests to go along with it.
• I have fixed a bug and have added a regression test.
• I have added a test using random numbers; I have made sure it uses a seed, and that random numbers generated are valid inputs for the tests.

[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

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[StellarBot]

Can one of the admins verify this patch?

[Copilot]

Pull request overview

Removes leftover stdexec-test guards and obsolete completion signature detection code, and updates async_mpi’s transform_mpi_* sender/receiver types to use the member-based P2300 receiver/sender shape instead of tag_invoke for set_*/connect.

Changes:

• Deleted dead stdexec-guarded tests/blocks in execution_base unit tests and removed the now-unused unit test target.
• Removed the obsolete detail::has_completion_signatures trait and its remaining test usage.
• Migrated async_mpi transform_mpi_receiver/transform_mpi_sender to member set_value/set_error/set_stopped and member connect (keeping tag_invoke(get_completion_signatures_t, ...)).

Reviewed changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
libs/core/execution_base/tests/unit/test_tag_invoke_only_completion_signatures.cpp	Removed a unit test file that was effectively disabled by stdexec configuration.
libs/core/execution_base/tests/unit/coroutine_utils.cpp	Removed dead stdexec-guarded assertions for the removed awaitable-as-sender path; added explanatory note.
libs/core/execution_base/tests/unit/completion_signatures.cpp	Removed assertion that depended on the deleted has_completion_signatures trait.
libs/core/execution_base/tests/unit/CMakeLists.txt	Removed the deleted test target from the unit test list.
libs/core/execution_base/include/hpx/execution_base/completion_signatures.hpp	Deleted the obsolete detail::has_completion_signatures trait.
libs/core/async_mpi/include/hpx/async_mpi/transform_mpi.hpp	Updated transform_mpi sender/receiver to member-based P2300 forms for set_* and connect.

[guptapratykshh]

pre-existing issue in the master

[hkaiser]

@guptapratykshh Can you please fix this while you're touching this code?