refactor(snomed): adapt shim — biodb retired the bulk downloader

[bschilder]

Summary

Follow-up to bioDB#18. The SNOMED bulk downloader (and the GitHub Release asset it pulled) was retired in bioDB for SNOMED CT licensing reasons — onward redistribution from a public mirror isn't permitted under the IHTSDO / UMLS license.

synthlab/download_snomed.py was a shim that re-exported the bioDB names. With those names removed upstream, the shim needs an update.

Changes

• Drops stale imports of GITHUB_*, SNOMED_RELEASE_URL, download_concept_csv — those names no longer exist in biodb.snomed.
• Re-exports the OLS-backed query helpers (query_concept, search_concepts, get_descendants, get_ancestors, get_children, get_parents) and the parsers (load_concept_csv, load_concept_csv_from_zip) from biodb.snomed. The OLS path is unaffected by the licensing change — EBI handles SNOMED CT licensing server-side.
• Keeps download_snomed_vocabulary / is_snomed_available / get_concept_csv_path as deprecated stubs that raise RuntimeError with migration guidance pointing at https://athena.ohdsi.org. They can't be transparently emulated since the GitHub asset is gone.

The four names imported by synthlab/__init__.py (download_snomed_vocabulary, get_concept_csv_path, is_snomed_available, get_snomed_data_dir) all remain importable — the first three raise on call, the last is the live biodb function.

Migration for downstream callers

# Was:
from synthlab.download_snomed import download_snomed_vocabulary
path = download_snomed_vocabulary()                            # → RuntimeError now

# Is:
from biodb.snomed import load_concept_csv
# 1. Get a bundle from https://athena.ohdsi.org (accept the SNOMED CT license)
df = load_concept_csv("~/Downloads/CONCEPT.csv")
# or
from biodb.snomed import load_concept_csv_from_zip
df = load_concept_csv_from_zip("~/Downloads/vocabulary_download_v5_xxx.zip")

Test plan

• Local import of from synthlab.download_snomed import ... resolves cleanly with biodb installed
• Calling any of the three deprecated stubs raises RuntimeError with the Athena migration note
• CI green (synthlab's pytest suite — exercises real install of biodb from git)

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