fix(15): correct API response field name (chats not messages)

The chat history API returns { chats: [...], next: ... } but the
fetchChatHistory.fulfilled handler was expecting { messages: [...] }.

This caused a TypeError when opening the chat window because
`messages` was undefined.

Fixed:
- sessionChatSlice.js: Extract `chats` from payload, default to []
- Updated all test payloads to use `chats` field name

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

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

@@ -537,7 +537,7 @@ describe('fetchChatHistory async thunk', () => {
       meta: { arg: { channel: 'session-abc' } },
       payload: {
         channel: 'session-abc',
-        messages: [
+        chats: [
           { id: 'msg-1', message: 'Hello', sender_id: 'user-1', created_at: '2026-01-26T12:00:00Z' }
         ],
         next: 20
@@ -576,7 +576,7 @@ describe('fetchChatHistory async thunk', () => {
       meta: { arg: { channel: 'session-abc' } },
       payload: {
         channel: 'session-abc',
-        messages: [
+        chats: [
           { id: 'msg-1', user_id: 'user-1', user: { name: 'User' }, message: 'Hello', created_at: '2026-01-26T12:00:00Z', channel: 'session' }, // Duplicate
           { id: 'msg-2', user_id: 'user-2', user: { name: 'User2' }, message: 'World', created_at: '2026-01-26T12:01:00Z', channel: 'session' }  // New
         ],
@@ -609,7 +609,7 @@ describe('fetchChatHistory async thunk', () => {
       meta: { arg: { channel: 'session-abc', before: 3 } },
       payload: {
         channel: 'session-abc',
-        messages: [
+        chats: [
           { id: 'msg-1', message: 'Oldest', createdAt: '2026-01-26T12:00:00Z' },
           { id: 'msg-2', message: 'Middle', createdAt: '2026-01-26T12:01:00Z' }
         ],
@@ -648,7 +648,7 @@ describe('fetchChatHistory async thunk', () => {
       meta: { arg: { channel: 'session-abc' } },
       payload: {
         channel: 'session-abc',
-        messages: [
+        chats: [
           { id: 'msg-1', message: 'Hello', createdAt: '2026-01-26T12:00:00Z' }
         ],
         next: null
@@ -674,7 +674,7 @@ describe('fetchChatHistory async thunk', () => {
       meta: { arg: { channel: 'session-abc' } },
       payload: {
         channel: 'session-abc',
-        messages: [
+        chats: [
           {
             id: 'msg-1',
             user_id: 'user-1',

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

@@ -306,7 +306,9 @@ const sessionChatSlice = createSlice({
       // fetchChatHistory fulfilled
       .addCase(fetchChatHistory.fulfilled, (state, action) => {
         const channel = action.meta.arg.channel;
-        const { messages, next } = action.payload;
+        // API returns { chats: [...], next: ... }
+        const { chats, next } = action.payload;
+        const messages = chats || [];
 
         // Initialize channel if not exists
         if (!state.messagesByChannel[channel]) {