Add a range constructor to vecsim_stl::vector

[dor-forer]

Describe the changes in the pull request

vecsim_stl::vector exposes only three constructors: empty, sized, and sized-with-fill-value. Building one from an existing range therefore means constructing it empty and copying the elements in by hand.

This adds the missing iterator-pair constructor, forwarding to the underlying std::vector and keeping the allocator as the trailing argument like the existing constructors:

template <std::input_iterator Iter>
explicit vector(Iter first, Iter last, const std::shared_ptr<VecSimAllocator> &alloc)
    : VecsimBaseObject(alloc), std::vector<T, VecsimSTLAllocator<T>>(first, last, alloc) {}

<iterator> is now included explicitly rather than relied upon transitively.

Why the std::input_iterator constraint: for a call like vecsim_stl::vector<size_t> v(10, 5, alloc) an unconstrained template would win overload resolution against the (count, value, alloc) constructor, since Iter = int is an exact match for both arguments while the fill constructor needs an int to size_t conversion, and conversion-sequence ranking is applied before the prefer-non-template tiebreak. In practice libstdc++ recovers (its own range constructor is SFINAE-guarded, so the forwarded (int, int, alloc) call lands back on its fill constructor), but relying on that is fragile and produces confusing diagnostics for genuinely bad calls. Constraining on std::input_iterator is the same idiom std::vector itself uses, keeps resolution correct at our own signature, and is free. No behaviour change for any existing call site.

Which issues this PR fixes

None. No MOD ticket: this came out of noticing the missing constructor. Happy to attach one if the convention requires it.

Main objects this PR modified

1. vecsim_stl::vector (src/VecSim/utils/vecsim_stl.h) - added the range constructor.
2. AllocatorTest (tests/unit/test_allocator.cpp) - new test_vector_range_constructor.

Mark if applicable

• This PR introduces API changes
• This PR introduces serialization changes

Neither box applies: the addition is internal C++ (not the extern "C" API in vec_sim.h), purely additive, and touches no serialized state.

Testing

New AllocatorTest.test_vector_range_constructor covers:

• a random access iterator range, asserting the data is allocated through VecSimAllocator in a single allocation of the exact size, and released on destruction
• a range taken from another vecsim_stl::vector
• a std::list range (non-contiguous iterators, size not known upfront)
• that vector<size_t>(10, 5, alloc) still resolves to the fill constructor

Verified on an Intel Xeon Platinum 8375C box (gcc 13.3, AVX512, SVS enabled):

• full make build clean, zero errors
• full make unit_test: 2607/2607 passed (the 8 reported as skipped are the pre-existing SVS quantization / dynamic-info-iterator cases, unrelated to this change)
• targeted run after adding the test: ctest -R Allocator 8/8 passed, including the new case
• make check-format clean

Note on scope

Nothing in src/ calls the new constructor yet. The nearest existing use is hnsw.h:915 in mutuallyConnectNewElement, where a fresh candidatesList is immediately filled by insert(end(), top_candidates.begin(), top_candidates.end()); converting it is a one-line readability change with identical allocation behaviour, and I left it out of this PR to keep a hot path out of scope. The first real consumer is likely the SQ8-with-norm path, whose QuantPreprocessor constructor already takes a const vecsim_stl::vector<float> &mean_vec that no src/ caller builds yet.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.15%. Comparing base (efd63da) to head (abb18bf).

Additional details and impacted files

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##             main    #1008   +/-   ##
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  Coverage   97.15%   97.15%           
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  Files         141      141           
  Lines        8328     8330    +2     
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+ Hits         8091     8093    +2     
  Misses        237      237           

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