Improve Qwen3 Coder XML parser: fix atomic_tag usage and operator precedence
- Inline literals inside atomic_tag to prevent nesting child tags
- Change from << to + operator to avoid optional whitespace between tokens
- Remove Tag::TOOL wrapper that was hiding child tags from mapper
Note: This addresses structural issues but tool extraction still fails for
tool-auto-single, tool-required-only, and parallel-tool-calls scenarios.
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Co-Authored-By: Claude <noreply@anthropic.com>