fix: remove pandas.concat signature hook

[kitagry]

Hi maintainers! Thank you for nice project!

Summary

Fixes #2124 - Remove pandas.concat signature hook that causes mypy to crash

Problem

MyPy 1.17.1+ crashes with AttributeError: 'Instance' object has no attribute 'items' when analyzing code that uses pd.concat(). This occurs because:

1. pandas-stubs 2.3.2.250827 now returns either Instance or UnionType for concat's first argument
2. Our pandas_concat_callback hook assumes the argument type is UnionType and tries to access its .items attribute
3. This assumption is no longer valid with the updated pandas-stubs

Solution

Remove the pandas_concat_callback hook and its associated get_function_signature_hook method entirely because:

• In update mypy plugin and tests #1007, we removed PANDAS_CONCAT_FALSE_POSITIVES workarounds since pandas-stubs (now officially maintained by pandas-dev) provides correct types
• The signature manipulation is no longer necessary with modern pandas-stubs
• This aligns with the project's direction to rely on official pandas type stubs rather than custom workarounds

Changes

• Remove get_function_signature_hook method from PanderaPlugin
• Remove pandas_concat_callback method
• Add return code validation in tests to catch mypy failures

Testing

• Tested locally with mypy 1.18.2 to verify the fix resolves the crash
• Note: The project dependencies keep mypy at 1.10.0 because upgrading mypy causes widespread type errors across the codebase that are outside the scope of this fix
• Existing mypy tests pass without crashes

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.48%. Comparing base (812b2a8) to head (c4a20a7).
⚠️ Report is 387 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2173       +/-   ##
===========================================
- Coverage   94.28%   81.48%   -12.80%     
===========================================
  Files          91      137       +46     
  Lines        7013    10871     +3858     
===========================================
+ Hits         6612     8858     +2246     
- Misses        401     2013     +1612     

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[cosmicBboy]

thanks @kitagry ! 🚀