GCC 13.4 + MacOS fails on CI

[foxtran]

Summary of Failures:

 8/33 xtb:unit / gfn0               FAIL              0.51s   exit status 1
 9/33 xtb:unit / gfn1               FAIL              0.91s   exit status 1
10/33 xtb:unit / gfn2               FAIL              0.97s   exit status 1
33/33 xtb:unit / gfnff              FAIL              8.40s   exit status 1

Ok:                 28  
Expected Fail:      1   
Fail:               4   
Unexpected Pass:    0   
Skipped:            0   
Timeout:            0  

For GFN-FF test, the output is following:

  Starting mindless-solvation ... (5/10)
       ... mindless-solvation [FAILED]
  Message: Condition not fullfilled
  Starting scaleup ... (6/10)
       ... scaleup [FAILED]
  Message: Condition not fullfilled

See full log here: https://github.com/grimme-lab/xtb/actions/runs/16020629275/job/45196519233

GCC 13.4 was released on June 5, 2025 and updated on MacOS runners some time ago. GCC 13.3 worked fine.

[foxtran]

The problem comes from implicit copy initialization just like what I've fixed in #1292.

So, for all types, the assignment operator should be implemented.