Implement P2300 stopped_as_optional sender adapter

[shivansh023023]

Implement P2300 stopped_as_optional sender adapter

Description

This PR implements the stopped_as_optional sender adapter as defined in the P2300 (Sender/Receiver) standard. This utility is crucial for handling pipeline cancellations gracefully by converting a "stopped" signal into a successful "value" signal containing a std::nullopt.

Technical Implementation:

• Three-Channel Mapping:
  • Value Channel: Wraps the result T into std::optional<T>.
  • Stopped Channel: Intercepted and rerouted to set_value(std::nullopt).
  • Error Channel: Passed through to the next receiver without modification.
• Lifecycle & Metadata: Correctly sets sends_stopped = false since the stopped signal is consumed by the adapter.
• Environment Support: Implements get_env forwarding to ensure context and completion schedulers are preserved across the pipeline.
• Syntax: Full support for both functional calling and the pipe (|) operator via partial_algorithm.

Fixes #

Proposed Changes

• Added libs/core/execution/include/hpx/execution/algorithms/stopped_as_optional.hpp.
• Added libs/core/execution/tests/unit/algorithm_stopped_as_optional.cpp covering value, error, and stopped scenarios.
• Registered the new unit test in libs/core/execution/tests/unit/CMakeLists.txt.

Background context

Currently, HPX's P2300 implementation requires manual handling of stopped signals which can lead to complex and verbose pipeline logic. Adding stopped_as_optional provides a standardized, ergonomic way to treat cancellation as a state rather than a control-flow break.

Checklist

• I have added a new feature and have added tests to go along with it.
• I have verified the build and runtime success locally (Exit Code 0).
• I have maintained original copyright headers and ensured alphabetical include ordering for inspect tool compliance.

[codacy-production]

Up to standards ✅

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