chat: migrate gpt_oss to modular PEG parser
Move GPT-OSS tool call parsing to common/chat-syntax/gpt-oss.cpp.
Uses native PEG parser for channel-based format with special tokens:
<|channel|>analysis|commentary|final, <|message|>...<|end|>
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
