Use concepts for push_back detection

[Boulea7]

Summary

• use a requires expression for push_back detection when concepts are enabled
• keep the C++17 detector fallback, with both paths checking the lvalue container passed to detail::insert
• add a Clang C++23 regression test using an explicit-object-parameter attribute

Fixes #225

Verification

• Clang 19/C++23: the explicit-object-parameter regression failed before the change and passes after it
• compile tests: GCC 12 and Clang 19 with C++17, C++20, and C++23
• runtime tests: GCC 12/C++17 and Clang 19/C++20 (32 tests each)
• CMake/Ninja with GCC 12/C++17: 32/32 CTest tests