peg-parser: add tests for preserved() token-aware matching
Add comprehensive tests for the preserved parser:
- Text fallback (success, fail, partial)
- Token-aware matching when token info available
- Token mismatch detection
- Sequence with preserved tokens
- Qwen3-Coder-like parser demo (with and without token info)
The tests demonstrate how preserved() enables parsers to match
special tokens at the token level when token boundary info is
available, falling back to text matching otherwise.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
