[BUG]: pybind11 should not expect there is always a shared_ptr when detecting enable_shared_from_this

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Problem description

Currently, when pybind11 detects a class inheriting std::enable_shared_from_this, it asserts users always uses std::shared_ptr to hold it. Specifically, it checks std::shared_ptr::element_type at this line:

https://github.com/pybind/pybind11/blob/47079b9e7b81a4b6d93c46a452115d7d8fd3488f/include/pybind11/pybind11.h#L1784

However, even when a class inherits std::enable_shared_from_this, an object of its does not necessarily be held in a std::shared_ptr. It can be held by std::shared_ptr in just some part of the code, and held by other holders when passing to pybind11. In my case, I warped around std::shared_ptr to build my own holder, which does not have an element_type member type.

A possible fix is to check the type of holder_type first before assuming it is a std::shared_ptr.

Reproducible example code

No response

Start at include/pybind11/pybind11.h around line 1784, where enable_shared_from_this handling inspects the holder type. Trace the surrounding holder and shared_ptr detection logic, then verify that custom holders without an element_type are handled without the assertion; add or update regression coverage if the repository's nearby tests expose this path.