solves #726: bug in default inverse mapping

[0Navin0]

This PR supersedes #727 (moved to a feature branch instead of main to follow repo workflow).

# =============================================
# Following the same example described in the issue #726  
# after bug fix
# =============================================

In [23]: wa
Out[23]:
<SkyCoord (ICRS): (ra, dec) in deg
    (5.46165002, -72.12183946)>


In [29]: wcsobj.backward_transform
---------------------------------------------------------------------------
NotImplementedError                       Traceback (most recent call last)
File ~/Softwares/gwcs/gwcs/wcs/_pipeline.py:639, in Pipeline.backward_transform(self)
    638 try:
--> 639     backward = self.forward_transform.inverse
    640 except NotImplementedError as err:

File ~/anaconda3/envs/rdm_env_latest/lib/python3.14/site-packages/astropy/modeling/core.py:1383, in Model.inverse(self)
   1382 elif self._inverse is not None:
-> 1383     result = self._inverse()
   1384     if result is not NotImplemented:
.
.
.
.
NotImplementedError: No analytical or user-supplied inverse transform has been implemented for this model.

# Even when backward transform is not implemented in the original definition of the `wcsobj`, it should default successfully to `numerical_inverse` as shown below:
In [24]: wcsobj.world_to_pixel(wa)
Out[24]: (np.float64(3.999999999638021), np.float64(3.999999999540023))

In [25]: wcsobj.numerical_inverse(wa.ra.deg, wa.dec.deg)
Out[25]: (3.999999999638021, 3.999999999540023)

This PR addresses a bug that triggered when a backward_transform is not defined and default numerical_inverse needs to be used.

Tasks

News fragment change types:

• changes/729.bugfix.rst: fixes an issue

[mcara]

It needs a change log entry and a unit test.

[mcara]

Suggested change

	if sky2pix_proj.__class__.__name__.startswith("pix2sky"):
	if type(sky2pix_proj).__name__.startswith("Pix2Sky")