algorithms: expose Proj parameter in hpx::min/max/minmax_element CPOs

[aneek22112007-tech]

Summary

This PR exposes the Proj (projection) parameter in the public Customization Point Objects (CPOs) for hpx::min_element, hpx::max_element, and hpx::minmax_element.

Previously, while the internal detail layer supported projections, the public parallel CPO overloads hardcoded hpx::identity_v. This change brings these algorithms into alignment with the std::ranges design and ensures consistency with the existing hpx::ranges container algorithms.

Technical Details

CPO Updates

• Updated all six tag_fallback_invoke overloads for each algorithm in libs/core/algorithms/include/hpx/parallel/algorithms/minmax.hpp.
• Added a Proj template parameter (defaulting to hpx::identity) to support the standard projection API across all execution policies (including seq, par, and async variations).
• Integrated hpx::parallel::traits::is_projected_v and hpx::parallel::traits::is_indirect_callable_v constraints to ensure strict type checking and improved error diagnostics at the CPO boundary.

Type Deduction Fix

• Resolved a regression in element_type deduction within sequential_minmax_element_ind.
• The previous deduction logic using std::iter_value_t<FwdIter>& was insufficient for the parallel reduction phase, where FwdIter refers to an iterator over partition results rather than the base elements.
• Restored the decltype(HPX_INVOKE(proj, *it)) pattern to correctly resolve the proxy value type when merging partition results.

Testing

• Added three new regression test files: min_element_proj.cpp, max_element_proj.cpp, and minmax_element_proj.cpp.
• Test coverage includes:
  • Projecting member variables from custom structs/pairs.
  • Value transformations (e.g., negating values to invert min/max logic).
  • Verification of projection forwarding in seq(task) async policies.
  • Edge cases (empty ranges, single-element ranges, and all-equal elements).
• Confirmed that all existing min_element, max_element, and minmax_element unit tests pass.

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; it uses a seed and the generated numbers are valid.

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Up to standards ✅

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