Add dual-path testing for legacy and experimental parsers
- Add chat_parser_impl enum (LEGACY/EXPERIMENTAL) to control parser selection
- Run test_template_output_parsers twice: once for each parser implementation
- Guard PEG-specific tests (streaming, tool ID extraction) with impl check
- Add use_new_parsers flag to template inputs based on impl parameter
- Wrapper lambda 'test' avoids changing all test call sites
This ensures both legacy and experimental parser paths are tested,
with streaming tests only running for experimental PEG parsers.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
