VolcEngine's OpenAI-compatible gateway rejects requests when both
max_tokens and max_completion_tokens are present (the latter added
by openai-python SDK v2.x serialization). Set the flag so nanobot
sends max_completion_tokens instead of max_tokens for volcengine,
volcengine_coding_plan, and by extension byteplus variants.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
