test: cover retrieval metadata coercion

[vku2018]

Summary

• add direct unit coverage for stargraph.nodes.retrieval._coerce_metadata
• assert scalar metadata values, including booleans, are preserved
• assert dict/list/None values are dropped, empty input stays empty, and mixed input keeps only scalar entries
• no production code changes

Validation

• uv run pytest tests/unit/test_retrieval_coerce_metadata.py -o addopts="" -q
• uv run ruff check tests/unit/test_retrieval_coerce_metadata.py
• uv run pyright tests/unit/test_retrieval_coerce_metadata.py
• git diff --check

Closes #139