Deduplicate metadata download boilerplate

[Zethson]

Summary

The identical download-and-cache block (build the cache path, skip if the file is already present, _download from the pertpy example-data server) was repeated seven times in _cell_line.py and again in _moa.py and _drug.py.

This PR extracts it into a single MetaData._download_metadata staticmethod, sitting next to the existing _warn_unmatch helper on the shared superclass:

_BASE_URL = "https://exampledata.scverse.org/pertpy"

@staticmethod
def _download_metadata(file_name: str) -> Path:
    """Download a metadata file into scanpy's cache directory if absent and return its path."""
    path = Path(settings.cachedir) / file_name
    if not path.exists():
        _download(url=f"{MetaData._BASE_URL}/{file_name}", output_file_name=file_name,
                  output_path=settings.cachedir, block_size=4096)
    return path

Each _download_* method drops from ~12 lines to 2–3, and the base URL / cache-skip logic now lives in one place.

Details

• The pharmgkb branch in _drug.py stays inline on purpose: it downloads a zip and renames the extracted file, with the existence check on a different file than the one downloaded, so folding it into the generic helper would change behavior.
• Collapsed the threefold source dispatch in Drug.annotate / Drug.lookup now that the lazy-load is uniform.
• Removed the now-unused Path / settings / _download imports from _cell_line.py and _moa.py.

Net change: +92 / −159 in the metadata package.

Tests

Added network-free unit tests (tests/metadata/test_metadata.py) for the new helper: the existing metadata tests cover downloads end-to-end but require the network, and the refactor moved URL construction into one spot that now affects every download. The new tests mock _download and assert (a) the URL is built from the base and (b) a cached file is not re-fetched.

The full tests/metadata/ suite (12 tests, real downloads) passes locally, confirming the refactor is behavior-preserving across all sources.

🤖 Generated with Claude Code

[codecov-commenter]

Codecov Report

❌ Patch coverage is 78.78788% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 77.98%. Comparing base (473da92) to head (6512ca1).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
pertpy/metadata/_cell_line.py	60.00%	4 Missing ⚠️
pertpy/metadata/_drug.py	72.72%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1027      +/-   ##
==========================================
+ Coverage   77.95%   77.98%   +0.02%     
==========================================
  Files          50       50              
  Lines        6587     6554      -33     
==========================================
- Hits         5135     5111      -24     
+ Misses       1452     1443       -9     

Files with missing lines	Coverage Δ
pertpy/metadata/_metadata.py	74.19% <100.00%> (+12.28%)	⬆️
pertpy/metadata/_moa.py	74.28% <100.00%> (-3.77%)	⬇️
pertpy/metadata/_drug.py	84.78% <72.72%> (+0.52%)	⬆️
pertpy/metadata/_cell_line.py	55.11% <60.00%> (-1.25%)	⬇️

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