FEAT: Add PinyinConverter for Chinese Pinyin-mix transformations

[Nantha kumar (Nanduu24)]

Description

Adds a deterministic (no-LLM) text-to-text converter, PinyinConverter, that rewrites Chinese (Hanzi) characters as their Pinyin romanization. Pinyin mixing is a Chinese-specific adversarial text transformation described in recent Chinese LLM safety work such as CSSBench: Hanzi characters or spans are rewritten as full or abbreviated Pinyin while the text stays understandable to a Chinese-reading model. This can bypass keyword- and token-level safety filters that match on Hanzi rather than on romanized readings. I could not find an existing PyRIT converter that performs a Hanzi→Pinyin transformation (translation changes the language; character-noise and Unicode converters don't model romanization).

Behavior

• mode: how a converted Hanzi is rendered
  • full — full toneless reading (中 → zhong)
  • initial — first letter only (中 → z)
  • mixed — chooses full or initial independently per character
• proportion ([0.0, 1.0]): fraction of Hanzi to convert. Values below 1.0 leave the rest as Hanzi, producing mixed Hanzi/Pinyin text. Selection count is round(proportion * n_hanzi), positions chosen via the converter's seeded RNG.
• separator: optional string inserted between converted syllables (e.g. " " → zhong xin); no trailing separator is left.
• seed: reproducible selection and, in mixed mode, reproducible per-character rendering.
• Non-Hanzi characters (Latin, digits, punctuation, whitespace, emoji, CJK punctuation) always pass through unchanged.

Example:

full    : nihao，shijie！zheshiyigeceshi。
initial : nh，sj！zsygcs。
mixed   : nih，shij！zheshiyigeceshi。
partial : jin 天天qi 很hao wo men 出去玩ba   (mode=full, proportion=0.5, separator=" ")

Dependency

pypinyin (MIT, pure-Python) is added as an optional dependency (pip install pyrit[pinyin]) and imported lazily inside the converter, following the pattern used by other optional-dependency converters. It is also added to the all extra. Conversion tests are skipif-gated when pypinyin is not installed, so they run in the dev_all CI job and skip cleanly in the dev job.

Testing

• 15 new unit tests in tests/unit/converter/test_pinyin_converter.py covering each mode, proportion/mixing, separator handling, passthrough, reproducibility, and validation.
• Full converter unit suite passes locally (1513 passed, 34 skipped).
• ruff check, ruff format --check, and ty check all pass.

Closes #2647

[Nantha kumar (Nanduu24)]

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