Split field validation logic out of _parse_v2_response into a dedicated
_validate_parsed_fields helper. This brings _parse_v2_response from CC=11
(grade C) down to CC=3 (grade A). The extracted validator is CC=9 (grade B).
No grade-C functions remain.
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>