Raise a warning when an invalid after model validator function signature is raised

[Viicos]

Reverts #11957.

Fixes #12413.

The reported flawed signature from the original issue was using the following signature: (cls, values, info) -> .... However, #11957 resulted in the much more common (as per this GH search) (cls, values) -> ... signature (also not supposed to be accepted) to be confusingly broken (not rejected at build time, but the values argument is now the validation info).

According to this comment, the high usage of the invalid signature is probably due to V1 root validators changed to after model validators, without changing from a class method to an instance method. As such, I updated the migration guide to mention this change.

Change Summary

Related issue number

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

[github-actions]

Coverage report

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File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
pydantic
functional_validators.py
warnings.py
Project Total

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

[codspeed-hq]

CodSpeed Performance Report

Merging #12414 will degrade performances by 5.59%

_{Comparing revert-error-after-validator-classmethod (6732095) with main (31af2c5)}

Summary

❌ 1 regression
✅ 45 untouched

⚠️ Please fix the performance issues or acknowledge them on CodSpeed.

Benchmarks breakdown

	Benchmark	BASE	HEAD	Change
❌	test_model_validators_serializers	768.8 µs	814.3 µs	-5.59%