[tests] Add return-iterator compliance and edge-case regression tests for hpx::partial_sort

[aneek22112007-tech]

Summary

The existing partial_sort.cpp test suite verified only that the sorted prefix contained correct values, but never validated the return value of hpx::partial_sort across any execution policy. According to the C++ standard (and HPX documentation), partial_sort must return an iterator pointing to last -- this was completely untested.

Additionally, the following boundary conditions had zero coverage:

• Empty range (first == last)
• • middle == first (zero elements to sort)
• • middle == last (degenerates to full sort)
• • Single-element range
• • Already-sorted input

• Changes## Summary

The existing partial_sort.cpp test suite verified only that the sorted prefix contained correct values, but never validated the return value of hpx::partial_sort across any execution policy. According to the C++ standard (and HPX documentation), partial_sort must return an iterator pointing to last -- this was completely untested.

Additionally, the following boundary conditions had zero coverage:

• Empty range (first == last)
• middle == first (zero elements to sort)
• middle == last (degenerates to full sort)
• Single-element range
• Already-sorted input

Changes

• libs/core/algorithms/tests/unit/algorithms/partial_sort.cpp (test-only change)
  • Added result == B.end() assertions to all three existing test functions: sequential, parallel, and async variants.
  • Added new test_partial_sort_edge_cases() function covering all boundary conditions listed above across all policies (seq, par, par_unseq, and no-policy sequential).

Testing

All new tests validate both return value correctness and content correctness (where applicable). The code is clang-format clean (verified locally with --dry-run --Werror).

Why This Matters

Return-iterator compliance is a correctness guarantee of the C++ standard. Callers chaining operations on the returned iterator rely on it pointing to exactly last. Without this regression test, a future refactor could silently break this contract.-

• • libs/core/algorithms/tests/unit/algorithms/partial_sort.cpp (test-only change)
• • Added result == B.end() assertions to all three existing test functions: sequential, parallel, and async variants.
• • Added new test_partial_sort_edge_cases() function covering all boundary conditions listed above across all policies (seq, par, par_unseq, and no-policy sequential).

• Testing

• All new tests validate both return value correctness and content correctness (where applicable). The code is clang-format clean (verified locally with --dry-run --Werror).

• Why This Matters

• Return-iterator compliance is a correctness guarantee of the C++ standard. Callers chaining operations on the returned iterator rely on it pointing to exactly last. Without this regression test, a future refactor could silently break this contract.The existing partial_sort.cpp test suite verified only that the sorted prefix contained correct values, but never validated the return value of hpx::partial_sort across any execution policy. According to the C++ standard (and HPX documentation), partial_sort must return an iterator pointing to last

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.

[StellarBot]

Can one of the admins verify this patch?

[codacy-production]

Up to standards ✅

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[hkaiser]

LGTM, thanks!