fix: enforce type in feedback_config, reject unknown fields, add validation tests (#31802)

[kanishk2106]

Issue Fixed

Fixes langchain-ai/langchain#31802

This PR improves validation in FeedbackConfigModel to ensure stricter feedback configuration and better error handling.

Changes:

1. Enforce type field as a required field in FeedbackConfigModel.
2. Add validation to reject unknown fields using a model_validator before model creation.
3. Update test cases in test_feedback_config.py to cover:
4. Valid and invalid configurations
5. Missing type handling
6. Extra fields detection
7. Literal enforcement and category validation

Tests
All tests pass successfully:
pytest tests/evaluation/test_feedback_config.py -v
8 passed in 0.02s
All validation scenarios covered (valid, invalid, missing type, extra fields, literal enforcement).

Notes
This change is backward-compatible and designed to work seamlessly alongside existing pydantic.v1 usage in the repository.

[kanishk2106]

Hi, This PR is ready for review — would appreciate feedback from the maintainers!