- Transform nested music_notation object from REST API to flat attachment fields
- Map music_notation.id → attachmentId, file_name → attachmentName, attachment_type → attachmentType
- Include purpose field from API response root
- Set attachmentSize to null (not available from REST API, only WebSocket)
- Matches WebSocket message format for consistent JKChatMessage rendering
Tests:
- Add test for REST API attachment transformation
- Fix pre-existing test bugs: incorrect sendMessage.fulfilled payload format
- Fix pre-existing test bug: fetchChatHistory deduplication test used wrong format
- All 89 tests now pass (previously 3 failures)
Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>
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