test: add unit tests for PolicySchema validation

[imran-siddique]

The PolicySchema in packages/agent-compliance/schemas/policy_schema.py needs tests. Create packages/agent-compliance/schemas/test_policy_schema.py covering: valid policy passes, missing required field (id) raises PolicyValidationError, empty rules list accepted, invalid types rejected, version defaults to 1.0.0. Use pytest + the PolicyValidationError exception. See packages/agent-compliance/tests/ for test patterns.