fix(cmake): don't link SHARED modules against libpython in FindPython mode

[henryiii]

Our own test suite fails if using python-build-standalone (or any other statically linked Python). This is one option for a fix.

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Description

In FindPython mode, pybind11_add_module(... SHARED ...) routes through CMake's Python_add_library, which hard-links every non-MODULE library against Python::Python. An extension module must not link libpython on macOS: with a statically linked interpreter (uv / python-build-standalone), the linked libpython3.x.dylib loads as a second, uninitialized copy of the Python runtime, two-level namespace binds the module's C-API calls to it, and the import aborts with:

Fatal Python error: PyInterpreterState_Get: the function must be called with the GIL
held, ... but the GIL is released (the current Python thread state is NULL)

This is exactly what the installed_function case of test_cmake_build (the only case that uses SHARED) hits when the repo's documented cmake --workflow venv flow (uv-created venv) runs on macOS. Linux hides the same duplicate-runtime setup through ELF symbol interposition, and macOS CI uses dynamically linked python.org builds, so CI never sees it.

Fix, in pybind11NewTools.cmake:

• SHARED now calls plain add_library instead of Python_add_library, and
• links pybind11::module instead of pybind11::embed (only STATIC keeps pybind11::embed).

This matches what classic mode (pybind11Tools.cmake) has always done for SHARED, so it aligns the two code paths rather than inventing new semantics. Verified locally on macOS arm64 with a uv-managed CPython 3.13: the built module no longer links libpython, the previously-failing installed_function case passes, and all other test_cmake_build cases (including both embed cases) still pass.

Behavior notes:

• SHARED no longer requires the Development.Embed component and no longer records a libpython dependency. An application that links a SHARED pybind11 library into an embedding executable must link libpython itself — already the case in classic mode.
• SHARED + WITH_SOABI previously produced an author warning from FindPython ("MODULE only") and was a no-op; it now fails at configure because WITH_SOABI is no longer parsed out. Left unhandled to keep the change minimal.

Suggested changelog entry:

• pybind11_add_module in FindPython mode no longer links SHARED extension modules against libpython, matching classic mode; this fixes import crashes with statically linked interpreters (e.g. uv / python-build-standalone) on macOS.