test: annotate the merge enrichers fixture so pyrefly indexes its fields

[gabrielspadon]

pyrefly inferred the enrichers literal in test_published_upgrade_keeps_version_metadata_coherent as a list of tuple[str, dict[str, Collection[str]]], so enrichers[1][1]["fields"]["author"] read as indexing a str (bad-index at tests/test_regression.py:1584). Annotating it with the type merge_with_policy declares for that parameter, list[tuple[str, dict[str, Any]]], clears the only pyrefly error in the tree once the environment matches the locks.

The local environment was also behind: it still had bibtexparser 1.4.4 while requirements-*.lock pin 2.0.0, which produced two false missing-import errors until it was reinstalled from the locks the way CI installs them.

• pyrefly: 0 errors
• mypy citeforge/ main.py (the CI gate): no issues in 56 files
• the test passes