Remove policyengine-bundles contract CI

[anth-volk]

Fixes #503

Summary

• Remove the PR job that installed policyengine-bundles only to validate bundle metadata contracts.
• Remove the corresponding test-only import of policyengine_bundles from core build metadata tests.
• Keep core metadata tests focused on policyengine-core runtime/build metadata; certified bundle verification now belongs to policyengine.py.

Validation

• Not run locally; this is a narrow CI/test cleanup and no runtime code changed.