chat: migrate seed_oss to PEG parser
Move seed_oss format handler to chat-syntax/seed-oss.cpp using
the constructed PEG parser pattern (similar to nemotron-v3).
- Format: <seed:tool_call><function=name>...</function></seed:tool_call>
- Thinking: <seed:think>...</seed:think>
- Parameters: <parameter=key>value</parameter>
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
