FEAT: Add VigenereConverter

[diamond8658]

Description

Adds VigenereConverter, a classical polyalphabetic cipher converter that generalizes CaesarConverter using a repeating keyword instead of a fixed shift. Closes #2304.

Follows the existing CaesarConverter/AtbashConverter structure:

• __init__(self, *, key: str, append_description: bool = False), validates the key is non-empty and ASCII alphabetic
• _build_identifier() returns a ComponentIdentifier with key as a param
• convert_async encodes the prompt, with the same append_description behavior rendering vigenere_description.yaml
• Non-alphabetic characters pass through unchanged and don't consume a position in the key, matching Caesar's handling of digits and punctuation

vigenere_description.yaml cites Handa et al. (arXiv:2402.10601), the paper specifically covering word substitution cipher jailbreaking, rather than the CipherChat paper cited by Caesar/Atbash, since that paper doesn't cover Vigenère.

Per the discussion on #2304, this does not touch pyrit/scenario/scenarios/garak/encoding.py (garak has no corresponding probe) or FoundryTechnique (will be picked up when Foundry's converter set is refreshed separately).

Tests and Documentation

New tests/unit/converter/test_vigenere_converter.py, 13 tests covering basic encoding, case preservation, key case insensitivity, non-alphabetic passthrough (including non-ASCII alphabetic characters, which was a real bug caught during development, see note below), wraparound, append_description, and invalid key handling.

Added VigenereConverter to the existing parametrized fixtures in tests/unit/converter/test_converter.py alongside Caesar/Atbash.

Verified against tests/unit/registry/test_converter_registry.py, confirms the converter is discovered, correctly classified as non-LLM-based, and buildable through the registry.

pyrit/converter/__init__.py updated with the import and __all__ export.

doc/code/converters/1_text_to_text_converters.py updated with a demo line. Ran with jupytext --execute --to notebook doc/code/converters/1_text_to_text_converters.py, all cells including the new one execute cleanly.

Implementation note: the initial version used str.isalpha() to detect letters, which returns True for non-ASCII characters (accented letters, etc.) not present in the cipher alphabet, causing a crash. Fixed by checking ASCII letter membership explicitly. Caesar and Atbash avoid this because they use str.translate(), which passes through unmapped characters silently.

[diamond8658]

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