chat: migrate minimax_m2 to modular PEG parser
Move minimax_m2 tool call parser from chat.cpp to
common/chat-syntax/minimax-m2.cpp as a modular implementation.
The format uses XML-style parameters:
<minimax:tool_call><invoke name="func"><parameter name="key">value</parameter></invoke></minimax:tool_call>
with optional <think>...</think> reasoning blocks.
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
