test_peg_parser: pass impl parameter instead of forcing experimental
The test_peg_parser helper was always setting experimental_new_parsers=true,
which didn't match the impl parameter being used for other test decisions.
Now it respects the impl parameter like other test helpers.
This enables nemotron_v3:legacy which was previously failing.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>