test(07-02): add failing tests for fetchChatHistory async thunk

Add unit tests for fetchChatHistory extra reducers:
- Test pending state sets loading status
- Test fulfilled state adds messages and updates status
- Test message deduplication on fulfilled
- Test pagination: prepending older messages
- Test rejected state sets error
- Test null cursor handling

Tests fail as expected - async thunk not yet implemented.

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

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

@@ -488,6 +488,167 @@ describe('setWindowPosition', () => {
   });
 });
 
+describe('fetchChatHistory async thunk', () => {
+  // Import will be added in implementation phase
+  // This is the RED phase - tests should fail
+  let fetchChatHistory;
+
+  test('sets loading state on pending', () => {
+    // Mock the pending action type
+    const action = {
+      type: 'sessionChat/fetchChatHistory/pending',
+      meta: { arg: { channel: 'session-abc' } }
+    };
+    const state = {
+      fetchStatus: {},
+      fetchError: {},
+      messagesByChannel: {},
+      unreadCounts: {},
+      isWindowOpen: false,
+      activeChannel: null
+    };
+    const newState = sessionChatReducer(state, action);
+    expect(newState.fetchStatus['session-abc']).toBe('loading');
+    expect(newState.fetchError['session-abc']).toBeNull();
+  });
+
+  test('adds messages on fulfilled', () => {
+    const action = {
+      type: 'sessionChat/fetchChatHistory/fulfilled',
+      meta: { arg: { channel: 'session-abc' } },
+      payload: {
+        channel: 'session-abc',
+        messages: [
+          { id: 'msg-1', message: 'Hello', sender_id: 'user-1', created_at: '2026-01-26T12:00:00Z' }
+        ],
+        next: 20
+      }
+    };
+    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);
+    expect(newState.fetchStatus['session-abc']).toBe('succeeded');
+    expect(newState.nextCursors['session-abc']).toBe(20);
+  });
+
+  test('deduplicates messages on fulfilled', () => {
+    // Test that fetched messages don't duplicate existing ones
+    const state = {
+      messagesByChannel: {
+        'session-abc': [{ id: 'msg-1', message: 'Hello', createdAt: '2026-01-26T12:00:00Z' }]
+      },
+      fetchStatus: { 'session-abc': 'loading' },
+      fetchError: {},
+      nextCursors: {},
+      unreadCounts: {},
+      isWindowOpen: false,
+      activeChannel: null
+    };
+    const action = {
+      type: 'sessionChat/fetchChatHistory/fulfilled',
+      meta: { arg: { channel: 'session-abc' } },
+      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
+        ],
+        next: null
+      }
+    };
+    const newState = sessionChatReducer(state, action);
+    expect(newState.messagesByChannel['session-abc']).toHaveLength(2);
+    expect(newState.messagesByChannel['session-abc'][0].id).toBe('msg-1');
+    expect(newState.messagesByChannel['session-abc'][1].id).toBe('msg-2');
+  });
+
+  test('prepends older messages for pagination', () => {
+    // When fetching older messages, they should be prepended (oldest first)
+    const state = {
+      messagesByChannel: {
+        'session-abc': [
+          { id: 'msg-3', message: 'Newest', createdAt: '2026-01-26T12:02:00Z' }
+        ]
+      },
+      fetchStatus: { 'session-abc': 'loading' },
+      fetchError: {},
+      nextCursors: {},
+      unreadCounts: {},
+      isWindowOpen: false,
+      activeChannel: null
+    };
+    const action = {
+      type: 'sessionChat/fetchChatHistory/fulfilled',
+      meta: { arg: { channel: 'session-abc', before: 3 } },
+      payload: {
+        channel: 'session-abc',
+        messages: [
+          { id: 'msg-1', message: 'Oldest', createdAt: '2026-01-26T12:00:00Z' },
+          { id: 'msg-2', message: 'Middle', createdAt: '2026-01-26T12:01:00Z' }
+        ],
+        next: null
+      }
+    };
+    const newState = sessionChatReducer(state, action);
+    expect(newState.messagesByChannel['session-abc']).toHaveLength(3);
+    expect(newState.messagesByChannel['session-abc'][0].id).toBe('msg-1');
+    expect(newState.messagesByChannel['session-abc'][1].id).toBe('msg-2');
+    expect(newState.messagesByChannel['session-abc'][2].id).toBe('msg-3');
+  });
+
+  test('sets error state on rejected', () => {
+    const action = {
+      type: 'sessionChat/fetchChatHistory/rejected',
+      meta: { arg: { channel: 'session-abc' } },
+      error: { message: 'Not Found' }
+    };
+    const state = {
+      fetchStatus: { 'session-abc': 'loading' },
+      fetchError: {},
+      messagesByChannel: {},
+      unreadCounts: {},
+      isWindowOpen: false,
+      activeChannel: null
+    };
+    const newState = sessionChatReducer(state, action);
+    expect(newState.fetchStatus['session-abc']).toBe('failed');
+    expect(newState.fetchError['session-abc']).toBe('Not Found');
+  });
+
+  test('handles null next cursor', () => {
+    const action = {
+      type: 'sessionChat/fetchChatHistory/fulfilled',
+      meta: { arg: { channel: 'session-abc' } },
+      payload: {
+        channel: 'session-abc',
+        messages: [
+          { id: 'msg-1', message: 'Hello', createdAt: '2026-01-26T12:00:00Z' }
+        ],
+        next: null
+      }
+    };
+    const state = {
+      messagesByChannel: {},
+      fetchStatus: { 'session-abc': 'loading' },
+      fetchError: {},
+      nextCursors: {},
+      unreadCounts: {},
+      isWindowOpen: false,
+      activeChannel: null
+    };
+    const newState = sessionChatReducer(state, action);
+    expect(newState.nextCursors['session-abc']).toBeNull();
+  });
+});
+
 describe('sessionChat integration', () => {
   test('complete message flow: receive → set active → open window → mark read', () => {
     const initialState = {