Allow TRO fallback for pinned certified datasets

[MaxGhenis]

Summary

• Allow TRACE TRO generation to proceed when the country release manifest pins the certified dataset SHA but the separate data release manifest does not list that dataset
• Omit the stale/inapplicable data release manifest artifact in that fallback path
• Add regression coverage for the fallback

Tests

• uv run --locked --extra dev python -m pytest tests/test_trace_tro.py -q
• uv run --locked --extra dev python -m pytest tests/test_bundle_refresh.py tests/test_release_manifests.py tests/test_us_long_term_datasets.py -q
• uv run --locked ruff check src/policyengine/provenance/trace.py tests/test_trace_tro.py
• uv run --locked ruff format --check src/policyengine/provenance/trace.py tests/test_trace_tro.py
• git diff --check