fix: improve PEG mapper content handling and add stricter tag validation
- Base mapper now concatenates CONTENT/REASONING tags instead of overwriting
(fixes Kimi K2 content_before being lost when content_after is empty)
- Add stricter tag validation with explicit error for unexpected tags
- native_mapper: explicitly route REASONING/CONTENT/NONE to base mapper
- constructed_mapper: same tag routing, removes call to base mapper for all tags
- Remove unused has_tool_args_blob member from constructed_mapper
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>