feat(07-02): implement REST API chat methods getChatMessages and sendChatMessage

Implement chat API methods using native fetch:
- getChatMessages: Fetch messages with pagination support
- sendChatMessage: Send messages to session/global channels
- URL construction with URLSearchParams for clean query strings
- Error handling for all HTTP status codes
- Credentials and headers included for session auth
- JSDoc comments for API documentation

All 14 unit tests passing.

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

jam-ui/src/helpers/__tests__/rest.test.js

@@ -165,14 +165,16 @@ describe('sendChatMessage', () => {
     expect(fetch).toHaveBeenCalledWith(
       expect.stringContaining('/api/chat'),
       expect.objectContaining({
-        method: 'POST',
-        body: JSON.stringify({
-          channel: 'session',
-          session_id: 'session-abc',
-          message: 'Hello world'
-        })
+        method: 'POST'
       })
     );
+    // Check body content separately (key order doesn't matter)
+    const body = JSON.parse(fetch.mock.calls[0][1].body);
+    expect(body).toEqual({
+      channel: 'session',
+      session_id: 'session-abc',
+      message: 'Hello world'
+    });
     expect(result.message.id).toBe('msg-new');
   });
 

jam-ui/src/helpers/rest.js

@@ -952,3 +952,58 @@ export const openMetronome = options => {
       .catch(error => reject(error));
   });
 };
+
+/**
+ * Fetch chat messages for a channel
+ * @param {Object} params - Request parameters
+ * @param {string} params.channel - Channel type ('session', 'global', 'lesson')
+ * @param {string} [params.sessionId] - Session ID (required for session channel)
+ * @param {number} [params.before] - Pagination cursor
+ * @returns {Promise<{messages: Array, next: number|null}>} Message list with pagination
+ */
+export const getChatMessages = async ({ channel, sessionId, before }) => {
+  const params = new URLSearchParams({ channel });
+  if (sessionId) params.append('session_id', sessionId);
+  if (before !== undefined) params.append('before', before);
+
+  const baseUrl = process.env.REACT_APP_API_BASE_URL;
+  const response = await fetch(`${baseUrl}/chat?${params}`, {
+    method: 'GET',
+    credentials: 'include',
+    headers: { 'Content-Type': 'application/json' }
+  });
+
+  if (!response.ok) {
+    throw new Error(`${response.status} ${response.statusText}`);
+  }
+
+  return response.json();
+};
+
+/**
+ * Send a chat message to a channel
+ * @param {Object} params - Request parameters
+ * @param {string} params.channel - Channel type ('session', 'global', 'lesson')
+ * @param {string} [params.sessionId] - Session ID (required for session channel)
+ * @param {string} params.message - Message content
+ * @returns {Promise<{message: Object}>} Created message object
+ */
+export const sendChatMessage = async ({ channel, sessionId, message }) => {
+  const body = { channel, message };
+  if (sessionId) body.session_id = sessionId;
+
+  const baseUrl = process.env.REACT_APP_API_BASE_URL;
+  const response = await fetch(`${baseUrl}/chat`, {
+    method: 'POST',
+    credentials: 'include',
+    headers: { 'Content-Type': 'application/json' },
+    body: JSON.stringify(body)
+  });
+
+  if (!response.ok) {
+    const error = await response.json().catch(() => ({}));
+    throw new Error(error.error || `${response.status} ${response.statusText}`);
+  }
+
+  return response.json();
+};