Towards fixing scikit-image#7464

Currently, running the scikit-image tests fails on the free-threaded
build because the GIL is enabled at runtime:

```
_______________________________________________ ERROR collecting build-install/usr/lib/python3.14t/site-packages/skimage/feature/censure.py ________________________________________________
build-install/usr/lib/python3.14t/site-packages/skimage/feature/censure.py:6: in <module>
    from ..morphology import octagon, star
build-install/usr/lib/python3.14t/site-packages/skimage/morphology/__init__.py:28: in <module>
    from ._skeletonize import medial_axis, skeletonize, thin
build-install/usr/lib/python3.14t/site-packages/skimage/morphology/_skeletonize.py:10: in <module>
    from ._skeletonize_lee_cy import _compute_thin_image
E   RuntimeWarning: The global interpreter lock (GIL) has been enabled to load module 'skimage.morphology._skeletonize_lee_cy', which has not declared that it can run safely without the GIL. To override this behavior and keep the GIL disabled (at your own risk), run with PYTHON_GIL=0 or -Xgil=0.
```

This is happening because the `cython_args` aren't being passed to the
C++ codegen function in the project's meson configuration.

I changed the name of the variable to `cython_codegen_args` to make it a
little clearer it has nothing to do with `cython_c_args` and
`cython_cpp_args`, which are passed to e.g. clang and not Cython. I also
made `cython_gen_cpp` use the same arguments as `cython_gen`.

`c_undefined_ok` is unused, so I deleted it.

Finally, I added a regression test for this in the project's
`conftest.py`, following a similar check I added to numpy's
`conftest.py` a while back. I also added a 3.14t testing job and deleted
the 3.13t job and special testing logic. All of that is unnecessary now.

I will follow up after this to make the 3.14t testing job use
pytest-run-parallel, superseding scikit-image#7678.

* origin/main: (27 commits)
  Move `_shared` to `_skimage2` (#8102)
  Port `pad_footprint` & `mirror_footprint` to `skimage2` (#8094)
  Add rescaling API to skimage2 (#8075)
  Move `skimage2` implementation into `_skimage2` (#8093)
  Undo (double) mirroring in `ski2.morphology.dilation` (#8060)
  Grammar (#8091)
  Move `ensure_spacing` into `skimage2` (#8067)
  Ensure that `skimage2` does not eagerly import `skimage` (#8087)
  Use lazy-loader 0.5 (#8080)
  MAINT: make ellipse fitting forward compatible  (#8054)
  Avoid deprecated assign to ndarray.shape (#8020)
  Avoid circular import in `feature/corner.py` (#8077)
  Turn off coderabbit auto-labeling & label checks (#8070)
  Fix GIL being re-enabled by C++ extensions (#8059)
  Fix conventions following docstub and misc. (#8055)
  Port grayscale morphology operators to skimage2 (#8046)
  Add revised `peak_local_max` to `skimage2` (#8039)
  Allow read-only arrays as input to remap (#7535)
  Add `prescale`  parameter to "blob functions" (#7858)
  Try to make coderabbit respect exisiting type labels (#8042)
  ...