refactor(07-02): extract optimistic message creation helper

Extract createOptimisticMessage helper function:
- Improves code clarity and maintainability
- Centralizes optimistic message structure
- Makes testing easier if needed in future
- All tests still passing

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

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

@@ -89,6 +89,21 @@ const getChannelKey = (message) => {
   return message.channel; // 'global'
 };
 
+/**
+ * Helper function to create an optimistic message
+ * @param {Object} params - Message parameters
+ * @returns {Object} Optimistic message object
+ */
+const createOptimisticMessage = ({ optimisticId, userId, userName, message, channel }) => ({
+  id: optimisticId,
+  senderId: userId,
+  senderName: userName,
+  message,
+  createdAt: new Date().toISOString(),
+  channel,
+  isOptimistic: true
+});
+
 const sessionChatSlice = createSlice({
   name: 'sessionChat',
   initialState,
@@ -232,15 +247,14 @@ const sessionChatSlice = createSlice({
           state.messagesByChannel[channel] = [];
         }
 
-        state.messagesByChannel[channel].push({
-          id: optimisticId,
-          senderId: userId,
-          senderName: userName,
+        const optimisticMessage = createOptimisticMessage({
+          optimisticId,
+          userId,
+          userName,
           message,
-          createdAt: new Date().toISOString(),
-          channel,
-          isOptimistic: true
+          channel
         });
+        state.messagesByChannel[channel].push(optimisticMessage);
       })
       // sendMessage fulfilled - replace optimistic message
       .addCase(sendMessage.fulfilled, (state, action) => {