Fix: Silent Data Corruption in rna2vec due to Case Sensitivity

fix #606

📖 Summary

This PR resolves a critical bug in the rna2vec utility function where secondary structure sequences were processed in a case-sensitive manner. This caused any lowercase input to fail triplet matching silently, returning vectors populated entirely by zeros.

🔍 Technical Root Cause

The rna2vec function transforms biological sequences into numerical representations by extracting overlapping triplets and mapping them to indices via a pre-generated vocabulary.

• The Discrepancy: While RNA sequences were normalized through dna2rna(), secondary structure (SS) sequences were taken directly from the input.
• The Failure: The internal triplet dictionary is generated using uppercase characters (S, H, M, I, B, X, E). When a user provided a lowercase sequence like "sshh", the triplet lookup triplets.get("ssh", 0) would fail to find a match and default to 0 (the padding index).
• The Consequence: Instead of a meaningful numerical representation, the model would receive an all-zero vector, effectively "masking" valid data without warning the user.

🛠️ Proposed Changes

1. Robust Normalization

Updated the core loop in pyaptamer/utils/_rna.py to enforce .upper() normalization on every sequence at the start of the iteration. This ensures that:

• RNA sequences remain robust.
• Secondary structure sequences are now case-insensitive.
• Future sequence types added to the loop will inherit this safety by default.

2. Regression Testing

Introduced a new test file: pyaptamer/utils/tests/test_rna2vec_robustness.py.

• Test Case: Specifically validates that uppercase (SSHH) and lowercase (sshh) inputs yield identical, non-zero NumPy arrays.
• Verification: This ensures that valid structural information is correctly encoded regardless of the input's casing.

⚠️ Impact & Risks

• Data Integrity: This fix prevents "silent failures" where models appear to be training but are actually receiving empty data.
• Backward Compatibility: This is a non-breaking change. It only affects cases that were previously failing/returning zeros.

✅ Verification Results

• Unit Tests: pytest pyaptamer/utils/tests/test_rna2vec_robustness.py -> Passed.
• Existing Suite: pytest pyaptamer/utils/tests/test_rna.py -> Passed.
• Manual Verification: Confirmed that lowercase structural strings now map to their correct triplet indices in the vocabulary.