Tighten managed dataset fallback rules

[MaxGhenis]

Summary

• only fall back to bundled data certification when the data release manifest is unavailable, not on arbitrary fetch or parse failures
• preserve explicit unmanaged dataset URIs instead of silently rewriting them to local managed mirrors
• add regression tests for missing data manifests, fallback behavior, and unmanaged dataset source preservation

Testing

• PATH=/Users/maxghenis/PolicyEngine/policyengine.py/.venv/bin:$PATH /Users/maxghenis/PolicyEngine/policyengine.py/.venv/bin/ruff check src/policyengine/core/release_manifest.py src/policyengine/tax_benefit_models/us/model.py src/policyengine/tax_benefit_models/uk/model.py tests/test_release_manifests.py
• VIRTUAL_ENV=/Users/maxghenis/PolicyEngine/policyengine.py/.venv PATH=/Users/maxghenis/PolicyEngine/policyengine.py/.venv/bin:$PATH PYTHONPATH=src /Users/maxghenis/PolicyEngine/policyengine.py/.venv/bin/python -m pytest tests/test_release_manifests.py -q