chat: migrate qwen3_coder_xml to modular PEG parser
Move qwen3_coder_xml tool call parser from chat.cpp to
common/chat-syntax/qwen3-coder-xml.cpp as a modular implementation.
The format uses XML-style parameters:
<tool_call><function=name><parameter=key>value</parameter></function></tool_call>
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
