Fix outdated expected URL in SDF legacy path unit test

[Copilot]

test_sdf_url_legacy_version was asserting an OBO URL (/ontology/chebi.obo) as the expected result of _chebi_sdf_url() for a legacy version — a copy-paste error. The implementation in downloader.py was already correct.

Change

• tests/test_downloader.py: Updated the expected URL in test_sdf_url_legacy_version to the correct SDF path:

  …/archive/chebi_legacy/archive/rel230/SDF/ChEBI_complete.sdf.gz
  

  instead of the incorrect OBO path:

  …/archive/chebi_legacy/archive/rel230/ontology/chebi.obo
  

Original prompt

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This section details on the original issue you should resolve

<issue_title>Unit test fails for SDF legacy path</issue_title>
<issue_description>The currently implemented URL for downloading legacy SDF files is correct. The test is outdated.

@copilot please fix</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes Unit test fails for SDF legacy path #7

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