[test] Add projection tests for hpx::ranges::{unique_copy, partition_copy, remove_copy, remove_copy_if}

[aneek22112007-tech]

Proposed Changes

• Add test_unique_copy_projection() to unique_copy_range.cpp — exercises the Proj
  parameter of hpx::ranges::unique_copy across seq, par, par_unseq, seq(task),
  and par(task) policies using user_defined_type::val as the projected key,
  cross-checked against std::unique_copy
• Add test_partition_copy_projection() to partition_copy_range.cpp — exercises the
  Proj parameter of hpx::ranges::partition_copy with a simple integer threshold
  predicate applied to the projected val field, verified against std::partition_copy
  across all policies
• Add projected_element{key, tag} helper struct and test_remove_copy_projection() /
  test_remove_copy_if_projection() to remove_copy_range.cpp and
  remove_copy_if_range.cpp — exercises the Proj parameter of
  hpx::ranges::remove_copy and hpx::ranges::remove_copy_if across all execution
  policies including async task variants

Any background context you want to provide?

All four algorithms (unique_copy, partition_copy, remove_copy, remove_copy_if)
accept an optional Proj parameter in their CPO layer
(container_algorithms/unique.hpp, container_algorithms/remove_copy.hpp) and correctly
thread it to the internal dispatch. However, their existing range test files exercised
only the default hpx::identity projection, leaving the user-supplied projection
code path completely untested — a silent gap that would allow future refactors breaking
projection support to pass CI undetected.

This brings these algorithms in line with the projection testing standard already
established for is_partitioned (is_partitioned_projection_range.cpp), is_sorted,
and minmax_element.

No production code was changed. All additions are pure test code following existing
patterns (HPX_TEST, test::equal). clang-format --dry-run --Werror passes with
zero violations.

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]

@aneek22112007-tech some of the newly added tests fail to compile, could you please have a look? Also, inspect complains abut non-ASCII characters in the tests:

/libs/core/algorithms/tests/unit/container_algorithms/partition_copy_range.cpp:
    Non-ASCII: line 303
/libs/core/algorithms/tests/unit/container_algorithms/remove_copy_if_range.cpp:
    Non-ASCII: line 291
/libs/core/algorithms/tests/unit/container_algorithms/remove_copy_range.cpp:
    Non-ASCII: line 272
/libs/core/algorithms/tests/unit/container_algorithms/unique_copy_range.cpp:
    Non-ASCII: line 277