Fix `smart_holder` multiple/virtual inheritance bugs in `shared_ptr` and `unique_ptr` to-Python conversions (#5836)

* ChatGPT-generated diamond virtual-inheritance test case.

* Report "virtual base at offset 0" but don't skip test.

* Remove Left/Right virtual default dtors, to resolve clang-tidy errors:

```
/__w/pybind11/pybind11/tests/test_class_sh_mi_thunks.cpp:44:13: error: prefer using 'override' or (rarely) 'final' instead of 'virtual' [modernize-use-override,-warnings-as-errors]
   44 |     virtual ~Left() = default;
      |     ~~~~~~~ ^
      |                     override
/__w/pybind11/pybind11/tests/test_class_sh_mi_thunks.cpp:48:13: error: prefer using 'override' or (rarely) 'final' instead of 'virtual' [modernize-use-override,-warnings-as-errors]
   48 |     virtual ~Right() = default;
      |     ~~~~~~~ ^
      |                      override
```

* Add assert(ptr) in register_instance_impl, deregister_instance_impl

* Proper bug fix

* Also exercise smart_holder_from_unique_ptr

* [skip ci] ChatGPT-generated bug fix: smart_holder::from_unique_ptr()

* Exception-safe ownership transfer from unique_ptr to shared_ptr

ChatGPT:

* shared_ptr’s ctor can throw (control-block alloc). Using get() keeps unique_ptr owning the memory if that happens, so no leak.

* Only after the shared_ptr is successfully constructed do you release(), transferring ownership exactly once.

* [skip ci] Rename alias_ptr to mi_subobject_ptr to distinguish from trampoline code (which often uses the term "alias", too)

* [skip ci] Also exercise smart_holder::from_raw_ptr_take_ownership

* [skip ci] Add st.first comments (generated by ChatGPT)

* [skip ci] Copy and extend (raw_ptr, unique_ptr) reproducer from PR #5796

* Some polishing: comments, add back Left/Right dtors for consistency within test_class_sh_mi_thunks.cpp

* explicitly default copy/move for VBase to silence -Wdeprecated-copy-with-dtor

* Resolve clang-tidy error:

```
/__w/pybind11/pybind11/tests/test_class_sh_mi_thunks.cpp:67:5: error: 'auto ptr' can be declared as 'auto *ptr' [readability-qualified-auto,-warnings-as-errors]
   67 |     auto ptr = new Diamond;
      |     ^~~~
      |     auto *
```

* Expand comment in `smart_holder::from_unique_ptr()`

* Better Left/Right padding to make it more likely that we avoid "all at offset 0". Clarify comment.

* Give up on `alignas(16)` to resolve MSVC warning:

```
       "D:\a\pybind11\pybind11\build\ALL_BUILD.vcxproj" (default target) (1) ->
       "D:\a\pybind11\pybind11\build\tests\pybind11_tests.vcxproj" (default target) (13) ->
       (ClCompile target) ->
         D:\a\pybind11\pybind11\tests\test_class_sh_mi_thunks.cpp(70,17): warning C4316: 'test_class_sh_mi_thunks::Diamond': object allocated on the heap may not be aligned 16 [D:\a\pybind11\pybind11\build\tests\pybind11_tests.vcxproj]
         D:\a\pybind11\pybind11\tests\test_class_sh_mi_thunks.cpp(80,43): warning C4316: 'test_class_sh_mi_thunks::Diamond': object allocated on the heap may not be aligned 16 [D:\a\pybind11\pybind11\build\tests\pybind11_tests.vcxproj]
         C:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Tools\MSVC\14.44.35207\include\memory(2913,46): warning C4316: 'std::_Ref_count_obj2<_Ty>': object allocated on the heap may not be aligned 16 [D:\a\pybind11\pybind11\build\tests\pybind11_tests.vcxproj]
       C:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Tools\MSVC\14.44.35207\include\memory(2913,46): warning C4316:         with [D:\a\pybind11\pybind11\build\tests\pybind11_tests.vcxproj]
       C:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Tools\MSVC\14.44.35207\include\memory(2913,46): warning C4316:         [ [D:\a\pybind11\pybind11\build\tests\pybind11_tests.vcxproj]
       C:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Tools\MSVC\14.44.35207\include\memory(2913,46): warning C4316:             _Ty=test_class_sh_mi_thunks::Diamond [D:\a\pybind11\pybind11\build\tests\pybind11_tests.vcxproj]
       C:\Program Files\Microsoft Visual Studio\2022\Enterprise\VC\Tools\MSVC\14.44.35207\include\memory(2913,46): warning C4316:         ] [D:\a\pybind11\pybind11\build\tests\pybind11_tests.vcxproj]
         D:\a\pybind11\pybind11\include\pybind11\detail\init.h(77,21): warning C4316: 'test_class_sh_mi_thunks::Diamond': object allocated on the heap may not be aligned 16 [D:\a\pybind11\pybind11\build\tests\pybind11_tests.vcxproj]
```

The warning came from alignas(16) making Diamond over-aligned, while regular new/make_shared aren’t guaranteed to return 16-byte aligned memory on MSVC (hence C4316). I’ve removed the explicit alignment and switched to asymmetric payload sizes (char[4] vs char[24]), which still nudges MI layout without relying on over-alignment. This keeps the test goal and eliminates the warning across all MSVC builds. If we ever want to stress over-alignment explicitly, we can add aligned operator new/delete under __cpp_aligned_new, but that’s more than we need here.

* Rename test_virtual_base_at_offset_0() → test_virtual_base_not_at_offset_0() and replace pytest.skip() with assert. Add helpful comment for future maintainers.