Add tests for _is_smiles, _is_inchikey, and guess_type methods in CompoundIdentifier class

[musasizivictoria]

Thank you for taking your time to contribute to Ersilia, just a few checks before we proceed

• Have you followed the guidelines in our Contribution Guide
• Have you written new tests for your core changes, as applicable?
• Have you successfully ran tests with your changes locally?

Description

This commit introduces unit tests for the following methods in the CompoundIdentifier class:

1. _is_smiles(): Positive and negative cases, both when RDKit (Chem) is available and not available.
2. _is_inchikey(): Valid and invalid InChIKey tests to ensure correct identification.
3. guess_type(): Tests to validate various inputs, including:

• InChIKeys
• SMILES strings
• Unprocessable inputs (like None and UNPROCESSABLE_INPUT)
• Whitespace-only inputs (\n, \t, space)
• Non-character inputs (numbers, floats, and random Unicode)

Changes to be made

Added the following tests:

1. _is_smiles() positive and negative tests for RDKit presence and absence.
2. _is_inchikey() positive and negative tests for valid/invalid keys.
3. guess_type() tests covering:

• Valid InChIKeys
• Valid SMILES strings
• Unprocessable inputs
• Whitespace-only inputs
• Non-character inputs

Status

• Tests written and validated locally using pytest.
• All tests passing successfully.
  

To do

• Documentation

Related to #1319

[musasizivictoria]

@DhanshreeA kindly this is ready for review. Thanks

cc @Malikbadmus

[DhanshreeA]

Quite a number of good tests here, I need to resolve conflicts in the PR before merging.