feat(07-01): implement core reducers with message deduplication logic

GREEN phase of TDD for Task 2:
- Implement addMessageFromWebSocket with full logic:
  * getChannelKey helper for session vs global channel keys
  * Channel initialization on first message
  * Message deduplication by msg_id using Array.some()
  * Sort messages by createdAt after insertion
  * Unread increment: only if window closed OR viewing different channel
- Implement setActiveChannel: sets activeChannel + channelType
- Implement openChatWindow: sets isWindowOpen, resets unread count, updates lastReadAt
- Implement closeChatWindow: sets isWindowOpen false only
- Export all 4 action creators

All 28 tests pass. Message deduplication validated.

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

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

@@ -31,13 +31,95 @@ const initialState = {
   windowPosition: null
 };
 
+/**
+ * Helper function to get channel key from message
+ * Session messages use 'session-{sessionId}', global uses 'global'
+ * @param {Object} message - Message object
+ * @returns {string} Channel key
+ */
+const getChannelKey = (message) => {
+  if (message.channel === 'session' && message.sessionId) {
+    return message.sessionId;
+  }
+  if (message.channel === 'lesson' && message.lessonSessionId) {
+    return message.lessonSessionId;
+  }
+  return message.channel; // 'global'
+};
+
 const sessionChatSlice = createSlice({
   name: 'sessionChat',
   initialState,
   reducers: {
-    // Reducers will be added in Tasks 2-3
+    /**
+     * Add message from WebSocket
+     * Handles deduplication, sorting, and unread count increment
+     */
+    addMessageFromWebSocket: (state, action) => {
+      const message = action.payload;
+      const channel = getChannelKey(message);
+
+      // Initialize channel if not exists
+      if (!state.messagesByChannel[channel]) {
+        state.messagesByChannel[channel] = [];
+      }
+
+      // Deduplicate by msg_id
+      const exists = state.messagesByChannel[channel].some(m => m.id === message.id);
+      if (exists) return;
+
+      // Add message
+      state.messagesByChannel[channel].push(message);
+
+      // Sort by createdAt ASC
+      state.messagesByChannel[channel].sort((a, b) =>
+        new Date(a.createdAt) - new Date(b.createdAt)
+      );
+
+      // Increment unread count if window closed OR viewing different channel
+      if (!state.isWindowOpen || state.activeChannel !== channel) {
+        state.unreadCounts[channel] = (state.unreadCounts[channel] || 0) + 1;
+      }
+    },
+
+    /**
+     * Set active channel for viewing
+     */
+    setActiveChannel: (state, action) => {
+      const { channel, channelType } = action.payload;
+      state.activeChannel = channel;
+      state.channelType = channelType;
+    },
+
+    /**
+     * Open chat window
+     * Resets unread count for active channel
+     */
+    openChatWindow: (state) => {
+      state.isWindowOpen = true;
+
+      // Reset unread count for active channel
+      if (state.activeChannel) {
+        state.unreadCounts[state.activeChannel] = 0;
+        state.lastReadAt[state.activeChannel] = new Date().toISOString();
+      }
+    },
+
+    /**
+     * Close chat window
+     * Does not affect unread counts
+     */
+    closeChatWindow: (state) => {
+      state.isWindowOpen = false;
+    }
   }
 });
 
-export const {} = sessionChatSlice.actions;
+export const {
+  addMessageFromWebSocket,
+  setActiveChannel,
+  openChatWindow,
+  closeChatWindow
+} = sessionChatSlice.actions;
+
 export default sessionChatSlice.reducer;