test(agent-runner): update formatter test for dropped <messages> envelope

fe2e881 (#2556) removed the <messages> wrapper from formatChatMessages
so the Claude Agent SDK calls the API instead of emitting a synthetic
stub, but poll-loop.test.ts still asserted the wrapper. The test has
failed on every PR built against main since. Assert the current shape:
no envelope, one self-contained <message> block per message.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: Adam

container/agent-runner/src/poll-loop.test.ts

@@ -37,13 +37,15 @@ describe('formatter', () => {
     expect(prompt).toContain('Hello world');
   });
 
-  it('should format multiple chat messages as XML block', () => {
+  it('should format multiple chat messages as distinct <message> blocks', () => {
     insertMessage('m1', 'chat', { sender: 'John', text: 'Hello' });
     insertMessage('m2', 'chat', { sender: 'Jane', text: 'Hi there' });
     const messages = getPendingMessages();
     const prompt = formatMessages(messages);
-    expect(prompt).toContain('<messages>');
-    expect(prompt).toContain('</messages>');
+    // The <messages> envelope was dropped in fe2e881b (#2556) so the SDK calls
+    // the API; each message is now its own self-contained <message> block.
+    expect(prompt).not.toContain('<messages>');
+    expect(prompt.match(/<message /g) ?? []).toHaveLength(2);
     expect(prompt).toContain('sender="John"');
     expect(prompt).toContain('sender="Jane"');
   });