[BUG] seq2vec silently drops unknown characters instead of mapping to index 0

[rupeshca007]

Reference Issues/PRs

Fixes #381

What does this implement/fix? Explain your changes.

Fixes a silent data corruption bug in seq2vec where unknown characters were completely dropped from sequences instead of being mapped to the 0 index token as documented.

Changes implemented:

• Added output.append(0) and output_ss.append(0) inside the unmatched logic block.
• • Implemented the seq_max_len boundary check in the fallback block to ensure chunk resets are correctly maintained.

What should a reviewer concentrate their feedback on?

• N/A

Did you add any tests for the change?

• No - existing tests continue to pass. The fix is a specific logic adjustment inside the tokenizer.

Any other comments?

None.

PR checklist

• The PR title starts with either [ENH], [MNT], [DOC], or [BUG].
• - [ ] Added/modified tests
• - [x] Used pre-commit hooks when committing to ensure that code is compliant with hooks.