feat(07-02): implement fetchChatHistory async thunk with extra reducers

Add fetchChatHistory async thunk with complete lifecycle handling:
- pending: Sets loading status and clears errors
- fulfilled: Adds messages with deduplication and sorting
- rejected: Sets error status with error message
- Message deduplication by ID to prevent duplicates
- Chronological sorting (createdAt ASC) after prepending
- Pagination cursor storage in nextCursors state
- Handles both session and global channels

All 46 unit tests passing.

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

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

@@ -1,4 +1,24 @@
-import { createSlice } from '@reduxjs/toolkit';
+import { createSlice, createAsyncThunk } from '@reduxjs/toolkit';
+import { getChatMessages } from '../../helpers/rest';
+
+/**
+ * Async thunk to fetch chat history for a channel
+ * @param {Object} params - Request parameters
+ * @param {string} params.channel - Channel ID (session ID, lesson ID, or 'global')
+ * @param {string} params.sessionId - Session ID for session channels
+ * @param {number} [params.before] - Pagination cursor
+ */
+export const fetchChatHistory = createAsyncThunk(
+  'sessionChat/fetchChatHistory',
+  async ({ channel, sessionId, before }) => {
+    const response = await getChatMessages({
+      channel: sessionId ? 'session' : 'global',
+      sessionId,
+      before
+    });
+    return { channel, ...response };
+  }
+);
 
 /**
  * Initial state for session chat
@@ -139,6 +159,46 @@ const sessionChatSlice = createSlice({
     setWindowPosition: (state, action) => {
       state.windowPosition = action.payload;
     }
+  },
+  extraReducers: (builder) => {
+    builder
+      // fetchChatHistory pending
+      .addCase(fetchChatHistory.pending, (state, action) => {
+        const channel = action.meta.arg.channel;
+        state.fetchStatus[channel] = 'loading';
+        state.fetchError[channel] = null;
+      })
+      // fetchChatHistory fulfilled
+      .addCase(fetchChatHistory.fulfilled, (state, action) => {
+        const channel = action.meta.arg.channel;
+        const { messages, next } = action.payload;
+
+        // Initialize channel if not exists
+        if (!state.messagesByChannel[channel]) {
+          state.messagesByChannel[channel] = [];
+        }
+
+        // Deduplicate messages
+        const existingIds = new Set(state.messagesByChannel[channel].map(m => m.id));
+        const newMessages = messages.filter(m => !existingIds.has(m.id));
+
+        // Prepend new messages (oldest first for pagination)
+        state.messagesByChannel[channel] = [...newMessages, ...state.messagesByChannel[channel]];
+
+        // Sort by createdAt ASC to maintain chronological order
+        state.messagesByChannel[channel].sort((a, b) =>
+          new Date(a.createdAt) - new Date(b.createdAt)
+        );
+
+        state.fetchStatus[channel] = 'succeeded';
+        state.nextCursors[channel] = next;
+      })
+      // fetchChatHistory rejected
+      .addCase(fetchChatHistory.rejected, (state, action) => {
+        const channel = action.meta.arg.channel;
+        state.fetchStatus[channel] = 'failed';
+        state.fetchError[channel] = action.error.message;
+      });
   }
 });