test(07-02): add failing tests for sendMessage async thunk with optimistic updates

Add unit tests for sendMessage optimistic UI flow:
- Test pending sets loading status
- Test pending adds optimistic message immediately
- Test pending initializes channel if needed
- Test fulfilled replaces optimistic message with real one
- Test rejected removes optimistic message
- Test other messages preserved during replace/remove

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

@@ -649,6 +649,216 @@ describe('fetchChatHistory async thunk', () => {
   });
 });
 
+describe('sendMessage async thunk', () => {
+  // Import will be added in implementation phase
+  // This is the RED phase - tests should fail
+  let sendMessage;
+
+  test('sets loading state on pending', () => {
+    const action = {
+      type: 'sessionChat/sendMessage/pending',
+      meta: {
+        arg: {
+          channel: 'session-abc',
+          message: 'Hello world',
+          optimisticId: 'temp-1',
+          userId: 'user-1',
+          userName: 'John Doe'
+        }
+      }
+    };
+    const state = {
+      sendStatus: 'idle',
+      sendError: null,
+      messagesByChannel: { 'session-abc': [] },
+      unreadCounts: {},
+      isWindowOpen: false,
+      activeChannel: null
+    };
+    const newState = sessionChatReducer(state, action);
+    expect(newState.sendStatus).toBe('loading');
+    expect(newState.sendError).toBeNull();
+  });
+
+  test('adds message optimistically on pending', () => {
+    const action = {
+      type: 'sessionChat/sendMessage/pending',
+      meta: {
+        arg: {
+          channel: 'session-abc',
+          message: 'Hello world',
+          optimisticId: 'temp-1',
+          userId: 'user-1',
+          userName: 'John Doe'
+        }
+      }
+    };
+    const state = {
+      messagesByChannel: { 'session-abc': [] },
+      sendStatus: 'idle',
+      sendError: null,
+      unreadCounts: {},
+      isWindowOpen: false,
+      activeChannel: null
+    };
+    const newState = sessionChatReducer(state, action);
+    expect(newState.messagesByChannel['session-abc']).toHaveLength(1);
+    expect(newState.messagesByChannel['session-abc'][0].id).toBe('temp-1');
+    expect(newState.messagesByChannel['session-abc'][0].message).toBe('Hello world');
+    expect(newState.messagesByChannel['session-abc'][0].senderId).toBe('user-1');
+    expect(newState.messagesByChannel['session-abc'][0].senderName).toBe('John Doe');
+    expect(newState.messagesByChannel['session-abc'][0].isOptimistic).toBe(true);
+  });
+
+  test('initializes channel if not exists on pending', () => {
+    const action = {
+      type: 'sessionChat/sendMessage/pending',
+      meta: {
+        arg: {
+          channel: 'new-channel',
+          message: 'First message',
+          optimisticId: 'temp-1',
+          userId: 'user-1',
+          userName: 'John'
+        }
+      }
+    };
+    const state = {
+      messagesByChannel: {},
+      sendStatus: 'idle',
+      sendError: null,
+      unreadCounts: {},
+      isWindowOpen: false,
+      activeChannel: null
+    };
+    const newState = sessionChatReducer(state, action);
+    expect(newState.messagesByChannel['new-channel']).toBeDefined();
+    expect(newState.messagesByChannel['new-channel']).toHaveLength(1);
+  });
+
+  test('replaces optimistic message with real one on fulfilled', () => {
+    const state = {
+      messagesByChannel: {
+        'session-abc': [
+          { id: 'temp-1', message: 'Hello world', senderId: 'user-1', isOptimistic: true }
+        ]
+      },
+      sendStatus: 'loading',
+      sendError: null,
+      unreadCounts: {},
+      isWindowOpen: false,
+      activeChannel: null
+    };
+    const action = {
+      type: 'sessionChat/sendMessage/fulfilled',
+      meta: {
+        arg: { optimisticId: 'temp-1', channel: 'session-abc' }
+      },
+      payload: {
+        message: {
+          id: 'msg-real',
+          message: 'Hello world',
+          sender_id: 'user-1',
+          sender_name: 'John Doe',
+          created_at: '2026-01-26T12:00:00Z',
+          channel: 'session'
+        }
+      }
+    };
+    const newState = sessionChatReducer(state, action);
+    expect(newState.messagesByChannel['session-abc']).toHaveLength(1);
+    expect(newState.messagesByChannel['session-abc'][0].id).toBe('msg-real');
+    expect(newState.messagesByChannel['session-abc'][0].isOptimistic).toBeUndefined();
+    expect(newState.sendStatus).toBe('succeeded');
+  });
+
+  test('removes optimistic message on rejected', () => {
+    const state = {
+      messagesByChannel: {
+        'session-abc': [
+          { id: 'temp-1', message: 'Hello world', senderId: 'user-1', isOptimistic: true }
+        ]
+      },
+      sendStatus: 'loading',
+      sendError: null,
+      unreadCounts: {},
+      isWindowOpen: false,
+      activeChannel: null
+    };
+    const action = {
+      type: 'sessionChat/sendMessage/rejected',
+      meta: {
+        arg: { optimisticId: 'temp-1', channel: 'session-abc' }
+      },
+      error: { message: 'Network error' }
+    };
+    const newState = sessionChatReducer(state, action);
+    expect(newState.messagesByChannel['session-abc']).toHaveLength(0);
+    expect(newState.sendStatus).toBe('failed');
+    expect(newState.sendError).toBe('Network error');
+  });
+
+  test('keeps other messages when replacing optimistic message', () => {
+    const state = {
+      messagesByChannel: {
+        'session-abc': [
+          { id: 'msg-1', message: 'Existing', createdAt: '2026-01-26T11:00:00Z' },
+          { id: 'temp-1', message: 'Optimistic', senderId: 'user-1', isOptimistic: true }
+        ]
+      },
+      sendStatus: 'loading',
+      sendError: null,
+      unreadCounts: {},
+      isWindowOpen: false,
+      activeChannel: null
+    };
+    const action = {
+      type: 'sessionChat/sendMessage/fulfilled',
+      meta: {
+        arg: { optimisticId: 'temp-1', channel: 'session-abc' }
+      },
+      payload: {
+        message: {
+          id: 'msg-real',
+          message: 'Optimistic',
+          sender_id: 'user-1',
+          created_at: '2026-01-26T12:00:00Z'
+        }
+      }
+    };
+    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-real');
+  });
+
+  test('keeps other messages when removing optimistic message', () => {
+    const state = {
+      messagesByChannel: {
+        'session-abc': [
+          { id: 'msg-1', message: 'Existing', createdAt: '2026-01-26T11:00:00Z' },
+          { id: 'temp-1', message: 'Failed', senderId: 'user-1', isOptimistic: true }
+        ]
+      },
+      sendStatus: 'loading',
+      sendError: null,
+      unreadCounts: {},
+      isWindowOpen: false,
+      activeChannel: null
+    };
+    const action = {
+      type: 'sessionChat/sendMessage/rejected',
+      meta: {
+        arg: { optimisticId: 'temp-1', channel: 'session-abc' }
+      },
+      error: { message: 'Failed to send' }
+    };
+    const newState = sessionChatReducer(state, action);
+    expect(newState.messagesByChannel['session-abc']).toHaveLength(1);
+    expect(newState.messagesByChannel['session-abc'][0].id).toBe('msg-1');
+  });
+});
+
 describe('sessionChat integration', () => {
   test('complete message flow: receive → set active → open window → mark read', () => {
     const initialState = {