Implement P2300 get_scheduler bridge for executors

[shivansh023023]

PR Description: Implement P2300 get_scheduler Bridge for HPX Executors

Fixes

This PR addresses the ongoing effort to align HPX Executors with the C++23 P2300 (std::execution) proposal.

Proposed Changes

• Added executor_scheduler adapter: Implemented a new bridge header libs/core/executors/include/hpx/executors/executor_scheduler.hpp that provides a P2300-compatible scheduler interface for existing HPX executors.
• Enabled get_scheduler for sequenced_executor: Added a tag_invoke overload for hpx::execution::experimental::get_scheduler in sequenced_executor.hpp, allowing it to be used directly in sender/receiver pipelines.
• Implemented Sender/Operation State Logic: Created the internal structures executor_scheduler_sender and executor_scheduler_operation_state to manage task dispatching via the executor.
• Added Unit Test: Introduced libs/core/executors/tests/unit/executor_scheduler.cpp to verify the end-to-end flow: schedule(get_scheduler(exec)) | then(...).

This work is part of the GSoC 2026 project: "Modernizing HPX Executors for P2300 Compatibility."

The implementation bridges the gap between traditional HPX executors and the newer sender/receiver paradigm. By providing an executor_scheduler adapter, we ensure that any standard HPX executor can be seamlessly integrated into P2300-style asynchronous pipelines.

Note: The implementation follows the architectural patterns established in the libs/core/executors module. While local linking encountered environment-specific hurdles regarding the __wrap_main entry point during the build process, the logic and header structures have been verified against the HPX core standards and are ready for CI validation.

Checklist

• 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 ✅

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[StellarBot]

Can one of the admins verify this patch?

[hkaiser]

@shivansh023023 This is really a very interesting PR connecting our executor infrastructure to senders&receivers. Thanks for this.

[hkaiser]

Could you please add get_scheduler_t overloads for the other executors as well?

[hkaiser]

Could you please use requires() instead of SFINAE:

Suggested change

	template <typename Exec,
	typename = std::enable_if_t<
	!std::is_same_v<std::decay_t<Exec>, executor_scheduler>>>
	explicit executor_scheduler(Exec&& exec)
	template <typename Exec>
	requires(!std::is_same_v<std::decay_t<Exec>, executor_scheduler>)
	explicit executor_scheduler(Exec&& exec)

[shivansh023023]

Thanks for the review, @hkaiser ! I've gone ahead and addressed your suggestions:

Modernization: Refactored the executor_scheduler constructor to use a C++20 requires clause, replacing the previous SFINAE/enable_if_t pattern.

Expanded Bridge: I’ve expanded the get_scheduler support across the executor ecosystem. This now includes parallel_executor (covering both policy variants) and restricted_thread_pool_executor.

Circular Dependencies: To avoid include loops with parallel_executor, I’ve implemented those specific tag_invoke overloads as free functions at the bottom of the header.

Verification: Updated the unit tests to verify scheduling through the bridge for these additional executors. Locally, executor_scheduler_test is passing cleanly.

[shivansh023023]

By the way @hkaiser Since I'm looking to prepare specifically for GSoC 2027 and want to become a long-term contributor to HPX, I was wondering if you could suggest any specific areas or sub-modules of the library that are currently high-priority or in need of modernization?

I'd love to start focusing my efforts on a specific domain now so I can build the necessary expertise to make a meaningful impact by the time the next GSoC cycle begins. Any guidance on where a student's focus would be most valuable would be greatly appreciated!
or can i create a thread on the discord channel to discuss about this ?