Add unit tests for cosmos client module (34 tests)

Coverage: get_pkn (format dict/dataframe/annnet, organism resolution,
categories, resources filtering), to_annnet (bulk vertices/edges,
entity types, edge attributes, polars/pandas), convenience functions
(categories, organisms, resources, status), URL override.

All HTTP calls mocked via patched Downloader. AnnNet tests use fake
module injection. No network or library dependencies required.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: deeenes