fix(tests): resolve 4 post-phase test failures

- platformClients.integration: iOS/Android push tests lacked
  setStatus() call before listNodes(), so platform filter excluded
  nodes. Added publishHeartbeat() to set platform on connection state.
- server.test: agent.send now emits run_state events before done (Phase
  1). Added sendAndWaitForDone() helper and updated test to find done
  event rather than assuming index 0.
- handlers.test: updated agent.send/cancel assertions to use find()
  and pass send arg to agent.cancel, consistent with run_state events.
- httpBody: req.destroy() closed socket before 413 response could be
  sent. Removed socket destruction from body reader; 413 responses now
  send Connection: close so Node closes the connection cleanly.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/gateway/ui/pages/chat.test.ts

@@ -248,8 +248,11 @@ describe('ChatPage wiring', () => {
     await Promise.resolve();
 
     const statusLine = Array.from(root.querySelectorAll('div'))
-      .find((el: any) => String(el.textContent ?? '').startsWith('Run status:'));
+      .find((el: any) => String(el.textContent ?? '').startsWith('Run status:')) as any;
     expect(statusLine).toBeTruthy();
+    if (!statusLine) {
+      throw new Error('Run status line not found');
+    }
     expect(statusLine.classList.contains('hidden')).toBe(false);
 
     resolveResult?.({ content: 'ok' });