Add nci_smiles_to_inchikey test

[tongyu0924]

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Description
This pull request adds new positive and negative tests for the _nci_smiles_to_inchikey function of the CompoundIdentifier class. These tests mock successful and failed API responses to ensure the function behaves correctly under different conditions.

Changes to be made

1. Added two new tests:
   • test_nci_smiles_to_inchikey_positive for valid SMILES to InChIKey conversion.
   • test_nci_smiles_to_inchikey_negative to test handling of invalid SMILES input with a 404 response.
2. Used @patch to mock API calls to ensure the tests don't rely on external dependencies.

Status

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To do

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Is this pull request related to any open issue? If yes, replace issueID below with the issue ID

Related to #1319

[DhanshreeA]

Closing this PR as redundant since these tests have already been merged.