[BUG] Raise ValueError for invalid return_type in encode_rna

[Alleny244]

Reference Issues/PRs

Fixes #566

What does this implement/fix? Explain your changes.

encode_rna() silently accepted invalid return_type values, leading to unexpected behavior downstream. This PR adds an upfront validation check that raises a ValueError with a clear message when return_type is not one of the supported values ("tensor" or "numpy"). The docstring is also updated to explicitly state the valid options.

What should a reviewer concentrate their feedback on?

• Whether the validation placement (before any processing) is appropriate
• Whether the error message is clear and actionable
• Whether the test parametrization covers sufficient edge cases (e.g. case sensitivity, empty string)

Did you add any tests for the change?

Yes:

• test_encode_rna_invalid_return_type — parametrized test verifying ValueError is raised for invalid inputs ("list", "array", "", "TENSOR", "Numpy")
• test_encode_rna_numpy_return_type — verifies the "numpy" return path produces an np.ndarray

Any other comments?

PR checklist

• The PR title starts with either [ENH], [MNT], [DOC], or [BUG]. [BUG] - bugfix, [MNT] - CI, test framework, [ENH] - adding or improving code, [DOC] - writing or improving documentation or docstrings.
• Added/modified tests
• Used pre-commit hooks when committing to ensure that code is compliant with hooks. Install hooks with pre-commit install.
  To run hooks independent of commit, execute pre-commit run --all-files