chat: migrate apriel_1_5 and xiaomi_mimo to modular PEG parser
Move apriel_1_5 and xiaomi_mimo tool call parsers from chat.cpp to
separate files in common/chat-syntax/ as modular implementations.
Apriel format: <tool_calls>[{"name": "...", "arguments": {...}}]</tool_calls>
with optional <thinking>...</thinking> reasoning.
XiaoMi MiMo format: <tool_call>{"name": "...", "arguments": {...}}</tool_call>
🤖 Generated with [Claude Code](https://claude.com/claude-code)
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>