Function test_container_shrink_to_fit

tests/test_container_helpers.h:685–706

// Test shrink_to_fit for heap-based containers
template<typename Container>
void test_container_shrink_to_fit() {
    Container c;

    // Add many elements
    for (int i = 0; i < 50; ++i) {
        populate(c, make_shared_int(i));
    }
    // Test capacity() - COMPILER ERROR if capacity() doesn't exist
    auto large_capacity = c.capacity();
    FL_CHECK(large_capacity > 50);

    // Remove most elements
    for (int i = 0; i < 40; ++i) {
        c.pop_back();
    }
    FL_CHECK(c.capacity() >= large_capacity);  // Capacity not reduced yet

    // Shrink - COMPILER ERROR if shrink_to_fit() doesn't exist
    c.shrink_to_fit();
    FL_CHECK(c.capacity() <= large_capacity);  // Should be reduced
    FL_CHECK(c.size() == 10);  // Size preserved
}

// Test max_size
template<typename Container>