feat(14-03): transform REST API music_notation to flat attachment fields

- 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>

jam-ui/src/store/features/__tests__/sessionChatSlice.test.js

@@ -577,8 +577,8 @@ describe('fetchChatHistory async thunk', () => {
       payload: {
         channel: 'session-abc',
         messages: [
-          { id: 'msg-1', message: 'Hello', createdAt: '2026-01-26T12:00:00Z' }, // Duplicate
-          { id: 'msg-2', message: 'World', createdAt: '2026-01-26T12:01:00Z' }  // New
+          { id: 'msg-1', user_id: 'user-1', user: { name: 'User' }, message: 'Hello', created_at: '2026-01-26T12:00:00Z', channel: 'session' }, // Duplicate
+          { id: 'msg-2', user_id: 'user-2', user: { name: 'User2' }, message: 'World', created_at: '2026-01-26T12:01:00Z', channel: 'session' }  // New
         ],
         next: null
       }
@@ -666,6 +666,52 @@ describe('fetchChatHistory async thunk', () => {
     const newState = sessionChatReducer(state, action);
     expect(newState.nextCursors['session-abc']).toBeNull();
   });
+
+  test('transforms music_notation nested object to attachment fields', () => {
+    // Mock REST API response with attachment message
+    const action = {
+      type: 'sessionChat/fetchChatHistory/fulfilled',
+      meta: { arg: { channel: 'session-abc' } },
+      payload: {
+        channel: 'session-abc',
+        messages: [
+          {
+            id: 'msg-1',
+            user_id: 'user-1',
+            user: { name: 'Test User' },
+            message: null,  // Attachment messages may have no text
+            created_at: '2024-01-15T12:00:00Z',
+            channel: 'session',
+            purpose: 'Notation File',
+            music_notation: {
+              id: 'notation-uuid-123',
+              file_name: 'sheet-music.pdf',
+              attachment_type: 'notation'
+            }
+          }
+        ],
+        next: null
+      }
+    };
+    const state = {
+      messagesByChannel: {},
+      fetchStatus: { 'session-abc': 'loading' },
+      fetchError: {},
+      nextCursors: {},
+      unreadCounts: {},
+      isWindowOpen: false,
+      activeChannel: null
+    };
+    const newState = sessionChatReducer(state, action);
+
+    expect(newState.messagesByChannel['session-abc']).toHaveLength(1);
+    const message = newState.messagesByChannel['session-abc'][0];
+    expect(message.attachmentId).toBe('notation-uuid-123');
+    expect(message.attachmentName).toBe('sheet-music.pdf');
+    expect(message.attachmentType).toBe('notation');
+    expect(message.purpose).toBe('Notation File');
+    expect(message.attachmentSize).toBeNull(); // Not available from REST API
+  });
 });
 
 describe('sendMessage async thunk', () => {
@@ -774,14 +820,12 @@ describe('sendMessage async thunk', () => {
         arg: { optimisticId: 'temp-1', channel: 'session-abc' }
       },
       payload: {
-        message: {
-          id: 'msg-real',
-          message: 'Hello world',
-          sender_id: 'user-1',
-          sender_name: 'John Doe',
-          created_at: '2026-01-26T12:00:00Z',
-          channel: 'session'
-        }
+        id: 'msg-real',
+        message: 'Hello world',
+        user_id: 'user-1',
+        user: { name: 'John Doe' },
+        created_at: '2026-01-26T12:00:00Z',
+        channel: 'session'
       }
     };
     const newState = sessionChatReducer(state, action);
@@ -837,12 +881,12 @@ describe('sendMessage async thunk', () => {
         arg: { optimisticId: 'temp-1', channel: 'session-abc' }
       },
       payload: {
-        message: {
-          id: 'msg-real',
-          message: 'Optimistic',
-          sender_id: 'user-1',
-          created_at: '2026-01-26T12:00:00Z'
-        }
+        id: 'msg-real',
+        message: 'Optimistic',
+        user_id: 'user-1',
+        user: { name: 'User 1' },
+        created_at: '2026-01-26T12:00:00Z',
+        channel: 'session'
       }
     };
     const newState = sessionChatReducer(state, action);

jam-ui/src/store/features/sessionChatSlice.js

@@ -307,15 +307,23 @@ const sessionChatSlice = createSlice({
         }
 
         // Transform API response format to internal format
-        // API returns: { user: { name: "..." }, user_id: "...", ... }
-        // Internal format: { senderName: "...", senderId: "...", ... }
+        // API returns: { user: { name: "..." }, user_id: "...", music_notation: { id, file_name, ... } }
+        // Internal format: { senderName: "...", senderId: "...", attachmentId: "...", ... }
+        // This matches the WebSocket message format (see JKSessionScreen.js handleChatMessage)
         const transformedMessages = messages.map(m => ({
           id: m.id,
           senderId: m.user_id,
           senderName: m.user?.name || 'Unknown',
           message: m.message,
           createdAt: m.created_at,
-          channel: m.channel
+          channel: m.channel,
+          // Attachment fields from REST API (music_notation nested object)
+          // Flatten to match WebSocket format for consistent rendering in JKChatMessage
+          purpose: m.purpose,
+          attachmentId: m.music_notation?.id,
+          attachmentName: m.music_notation?.file_name,
+          attachmentType: m.music_notation?.attachment_type,
+          attachmentSize: null  // Not available in REST API response (only in WebSocket)
         }));
 
         // Deduplicate messages