feat: add format-validation decorators for evaluators

[mvanhorn]

Summary

Evaluators had no reusable way to hard-fail malformed model output before scoring. Each evaluator re-implemented JSON parsing and format checks.

Changes

Add gepa.utils.format_validator with three decorators: require_json_output (parse JSON, optional shallow schema check, pass parsed value through), require_format (arbitrary predicate), and require_regex_match. On failure the wrapped evaluator is skipped and a (0.0, {feedback}) result is returned. Re-exported from gepa.utils.

Testing

Added tests/test_format_validator.py covering success, malformed JSON, schema mismatch, and regex paths.

Fixes #265

[semanticdiff-com]

Review changes with  

Changed Files

File	Status
src/gepa/utils/__init__.py	0% smaller
src/gepa/utils/format_validator.py	0% smaller
tests/test_format_validator.py	0% smaller

[mvanhorn]

Closing in favor of #386, which was opened a few minutes earlier and covers the same feature more completely (adds require_xml_output and docs). Left a note there.