test: add fuzz tests for policy YAML parsing

Add fuzz tests for YAML policy parsing in the fuzz/ directory. Use atheris (already configured in .clusterfuzzlite/). Create fuzz/fuzz_policy_schema.py that generates random YAML-like inputs and feeds them to PolicySchema.validate_policy(). Goal: find crashes, hangs, or unexpected exceptions from malformed policy files. Follow the pattern in existing fuzz/fuzz_policy_yaml.py.