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

• [x] Have you followed the guidelines in our Contribution Guide
• [x] Have you written new tests for your core changes, as applicable?
• [x] 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:

• [x] InChIKeys
• [x] SMILES strings
• [x] Unprocessable inputs (like None and UNPROCESSABLE_INPUT)
• [x] Whitespace-only inputs (\n, \t, space)
• [x] 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:

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

Status

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

To do

• [ ] Documentation

Related to https://github.com/ersilia-os/ersilia/issues/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.