fix: bind magic factories as method descriptors

[JosephDemarest]

Summary

• bind the decorated function when a MagicFactory is accessed through an instance
• preserve class-level access, factory options, widget_init, and the originating type map
• cover independent instance binding and ensure a temporary bound factory does not retain its instance

Fixes #733

Tests

• Python 3.12.9: python -m pytest -q (330 passed, 51 skipped, 13 xfailed)
• Python 3.14.6: python -m pytest tests/test_factory.py -q (12 passed)
• pre-commit run --all-files
• mypy src/magicgui/type_map/_magicgui.py

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.34%. Comparing base (2f08fb3) to head (9fb93ea).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #738      +/-   ##
==========================================
+ Coverage   89.32%   89.34%   +0.01%     
==========================================
  Files          40       40              
  Lines        4835     4843       +8     
==========================================
+ Hits         4319     4327       +8     
  Misses        516      516              

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