Add proper stacklevel for class-based config deprecation warning

[hannah-heywa]

Change Summary

Previously it showed up as originating here, but with stacklevel=4 we get it at the site of the issue e.g:

  .venv/lib/python3.13/site-packages/ninja/schema.py:181: PydanticDeprecatedSince20: Support for class-based `config` is deprecated, use ConfigDict instead. Deprecated in Pydantic V2.0 to be removed in V3.0. See Pydantic V2 Migration Guide at https://errors.pydantic.dev/2.11/migration/
    result = super().__new__(cls, name, bases, namespace, **kwargs)

.venv/lib/python3.13/site-packages/ninja/conf.py:8
.venv/lib/python3.13/site-packages/ninja_extra/conf/package_settings.py:63
...

Part of #7908.

Checklist

• The pull request title is a good summary of the changes - it will be used in the changelog
• Unit tests for the changes exist
• Tests pass on CI
• Documentation reflects the changes where applicable
• My PR is ready to review, please add a comment including the phrase "please review" to assign reviewers

[codspeed-hq]

CodSpeed Performance Report

Merging #12082 will not alter performance

_{Comparing hannah-heywa:patch-1 (3d9d40a) with main (ffc0840)}

Summary

✅ 46 untouched benchmarks

[github-actions]

Coverage report

Click to see where and how coverage changed

File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
pydantic/_internal
_config.py
Project Total

This report was generated by python-coverage-comment-action

[Viicos]

Thanks!