refactor(07-03): add JSDoc comments to CHAT_MESSAGE handler

- Add comprehensive JSDoc to getChannelKeyFromMessage helper
- Document channel key mapping logic (session, lesson, global)
- Add JSDoc to CHAT_MESSAGE handler with unread increment rules
- Extract shouldIncrementUnread for clarity
- All tests still passing after refactoring

Part of Phase 7 Plan 3 (WebSocket Integration & Selectors)

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

jam-ui/src/hooks/useSessionWebSocket.js

@@ -35,9 +35,17 @@ import {
 } from '../store/features/sessionChatSlice';
 
 /**
- * Helper function to get channel key from chat message
- * Session messages use sessionId directly, lesson uses lessonSessionId, global uses 'global'
+ * Helper function to get channel key from WebSocket chat message
+ *
+ * Channel keys map Protocol Buffer channel types to Redux state keys:
+ * - Session messages: Use session_id directly
+ * - Lesson messages: Use lesson_session_id directly
+ * - Global messages: Use 'global' literal
+ *
  * @param {Object} message - WebSocket message with channel and ID fields
+ * @param {string} message.channel - Channel type ('session', 'lesson', or 'global')
+ * @param {string} [message.session_id] - Session ID (for session channel)
+ * @param {string} [message.lesson_session_id] - Lesson session ID (for lesson channel)
  * @returns {string} Channel key for Redux state
  */
 const getChannelKeyFromMessage = (message) => {
@@ -187,7 +195,19 @@ export const useSessionWebSocket = (sessionId) => {
         }
       },
 
-      // Phase 7 Plan 3: Handle CHAT_MESSAGE from WebSocket
+      /**
+       * Phase 7 Plan 3: Handle CHAT_MESSAGE from WebSocket
+       *
+       * Transforms Protocol Buffer message format to Redux format and handles
+       * unread count increment based on chat window state.
+       *
+       * Unread increment logic:
+       * - Increment if window is closed
+       * - Increment if window is open but viewing a different channel
+       * - Do NOT increment if window is open and viewing the same channel
+       *
+       * @param {Object} message - Protocol Buffer formatted chat message
+       */
       CHAT_MESSAGE: (message) => {
         console.log('Chat message received:', message);
 
@@ -206,12 +226,13 @@ export const useSessionWebSocket = (sessionId) => {
           claimedRecording: message.claimed_recording || null
         };
 
-        // Add message to Redux
+        // Add message to Redux state
         dispatch(addMessageFromWebSocket(chatMessage));
 
-        // Increment unread count if window closed or different channel active
+        // Increment unread count if window closed or viewing different channel
         const messageChannelKey = getChannelKeyFromMessage(message);
-        if (!chatState.isWindowOpen || chatState.activeChannel !== messageChannelKey) {
+        const shouldIncrementUnread = !chatState.isWindowOpen || chatState.activeChannel !== messageChannelKey;
+        if (shouldIncrementUnread) {
           dispatch(incrementUnreadCount({ channel: messageChannelKey }));
         }
       },